Формат pdf как правильно пишется

Portable Document Format

Adobe PDF icon
Filename extension	.pdf
Internet media type	application/pdf,[1] application/x-pdf application/x-bzpdf application-gzpdf
Type code	PDF [1] (including a single space)
Uniform Type Identifier (UTI)	com.adobe.pdf
Magic number	%PDF
Developed by	Adobe Inc. (1991–2008) ISO (2008–)
Initial release	June 15, 1993; 29 years ago
Latest release	2.0
Extended to	PDF/A, PDF/E, PDF/UA, PDF/VT, PDF/X
Standard	ISO 32000-2
Open format?	Yes
Website	www.iso.org/standard/75839.html

Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems.^[2][3] Based on the PostScript language, each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, vector graphics, raster images and other information needed to display it. PDF has its roots in «The Camelot Project» initiated by Adobe co-founder John Warnock in 1991.^[4]

PDF was standardized as ISO 32000 in 2008.^[5] The last edition as ISO 32000-2:2020 was published in December 2020.

PDF files may contain a variety of content besides flat text and graphics including logical structuring elements, interactive elements such as annotations and form-fields, layers, rich media (including video content), three-dimensional objects using U3D or PRC, and various other data formats. The PDF specification also provides for encryption and digital signatures, file attachments, and metadata to enable workflows requiring these features.

History[edit]

Adobe Systems made the PDF specification available free of charge in 1993. In the early years PDF was popular mainly in desktop publishing workflows, and competed with a variety of formats such as DjVu, Envoy, Common Ground Digital Paper, Farallon Replica and even Adobe’s own PostScript format.

PDF was a proprietary format controlled by Adobe until it was released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO 32000-1:2008,^[6][7] at which time control of the specification passed to an ISO Committee of volunteer industry experts. In 2008, Adobe published a Public Patent License to ISO 32000-1 granting royalty-free rights for all patents owned by Adobe that are necessary to make, use, sell, and distribute PDF-compliant implementations.^[8]

PDF 1.7, the sixth edition of the PDF specification that became ISO 32000-1, includes some proprietary technologies defined only by Adobe, such as Adobe XML Forms Architecture (XFA) and JavaScript extension for Acrobat, which are referenced by ISO 32000-1 as normative and indispensable for the full implementation of the ISO 32000-1 specification.^[9] These proprietary technologies are not standardized and their specification is published only on Adobe’s website.^[10][11][12] Many of them are also not supported by popular third-party implementations of PDF.

In December 2020, the second edition of PDF 2.0, ISO 32000-2:2020, was published, including clarifications, corrections, and critical updates to normative references.^[13] ISO 32000-2 does not include any proprietary technologies as normative references.^[14]

Technical details[edit]

A PDF file is often a combination of vector graphics, text, and bitmap graphics. The basic types of content in a PDF are:

• Typeset text stored as content streams (i.e., not encoded in plain text);
• Vector graphics for illustrations and designs that consist of shapes and lines;
• Raster graphics for photographs and other types of images
• Multimedia objects in the document.

In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as a plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.

PDF combines three technologies:

• An equivalent subset of the PostScript page description programming language but in declarative form, for generating the layout and graphics.
• A font-embedding/replacement system to allow fonts to travel with the documents.
• A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.

PostScript language[edit]

PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. It can handle graphics and standard features of programming languages such as if statements and loop commands. PDF is largely based on PostScript but simplified to remove flow control features like these, while graphics commands equivalent to lineto remain.

Historically, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized.^{[clarification needed]} Any files, graphics, or fonts to which the document refers also are collected. Then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.^{[citation needed]}

As a document format, PDF has several advantages over PostScript:

• PDF contains tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
• PDF (since version 1.4) supports transparent graphics; PostScript does not.
• PostScript is an interpreted programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages in a PostScript document must be processed to determine the correct appearance of a given page, whereas each page in a PDF document is unaffected by the others. As a result, PDF viewers allow the user to quickly jump to the final pages of a long document, whereas a PostScript viewer needs to process all pages sequentially before being able to display the destination page (unless the optional PostScript Document Structuring Conventions have been carefully compiled and included).

PDF 1.6 and later supports interactive 3D documents embedded in a PDF file: 3D drawings can be embedded using U3D or PRC and various other data formats.^[15][16][17]

File format[edit]

A PDF file is organized using ASCII characters, except for certain elements that may have binary content.
The file starts with a header containing a magic number (as a readable string) and the version of the format, for example %PDF-1.7. The format is a subset of a COS («Carousel» Object Structure) format.^[18] A COS tree file consists primarily of objects, of which there are nine types:^[14]

• Boolean values, representing true or false
• Real numbers
• Integers
• Strings, enclosed within parentheses ((...)) or represented as hexadecimal within single angle brackets (<...>). Strings may contain 8-bit characters.
• Names, starting with a forward slash (/)
• Arrays, ordered collections of objects enclosed within square brackets ([...])
• Dictionaries, collections of objects indexed by names enclosed within double angle brackets (<<...>>)
• Streams, usually containing large amounts of optionally compressed binary data, preceded by a dictionary and enclosed between the stream and endstream keywords.
• The null object

Furthermore, there may be comments, introduced with the percent sign (%). Comments may contain 8-bit characters.

Objects may be either direct (embedded in another object) or indirect. Indirect objects are numbered with an object number and a generation number and defined between the obj and endobj keywords if residing in the document root. Beginning with PDF version 1.5, indirect objects (except other streams) may also be located in special streams known as object streams (marked /Type /ObjStm). This technique enables non-stream objects to have standard stream filters applied to them, reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF. Object streams do not support specifying an object’s generation number (other than 0).

An index table, also called the cross-reference table, is located near the end of the file and gives the byte offset of each indirect object from the start of the file.^[19] This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (incremental update). Before PDF version 1.5, the table would always be in a special ASCII format, be marked with the xref keyword, and follow the main body composed of indirect objects. Version 1.5 introduced optional cross-reference streams, which have the form of a standard stream object, possibly with filters applied. Such a stream may be used instead of the ASCII cross-reference table and contains the offsets and other information in binary format. The format is flexible in that it allows for integer width specification (using the /W array), so that for example, a document not exceeding 64 KiB in size may dedicate only 2  bytes for object offsets.

At the end of a PDF file is a footer containing

• The startxref keyword followed by an offset to the start of the cross-reference table (starting with the xref keyword) or the cross-reference stream object, followed by
• The %%EOF end-of-file marker.

If a cross-reference stream is not being used, the footer is preceded by the trailer keyword followed by a dictionary containing information that would otherwise be contained in the cross-reference stream object’s dictionary:

• A reference to the root object of the tree structure, also known as the catalog (/Root)
• The count of indirect objects in the cross-reference table (/Size)
• Other optional information

Within each page, there are one or multiple content streams that describe the text, vector and images being drawn on the page. The content stream is stack-based, similar to PostScript.^[20]

There are two layouts to the PDF files: non-linearized (not «optimized») and linearized («optimized»). Non-linearized PDF files can be smaller than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file. Linearized PDF files (also called «optimized» or «web optimized» PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since all objects required for the first page to display are optimally organized at the start of the file.^[21] PDF files may be optimized using Adobe Acrobat software or QPDF.

Imaging model[edit]

The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.

PDF graphics use a device-independent Cartesian coordinate system to describe the surface of a page. A PDF page description can use a matrix to scale, rotate, or skew graphical elements. A key concept in PDF is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description. PDF has (as of version 2.0) 25 graphics state properties, of which some of the most important are:

• The current transformation matrix (CTM), which determines the coordinate system
• The clipping path
• The color space
• The alpha constant, which is a key component of transparency
• Black point compensation control (introduced in PDF 2.0)

Vector graphics[edit]

As in PostScript, vector graphics in PDF are constructed with paths. Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text. Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves. Paths can be stroked, filled, fill then stroked, or used for clipping. Strokes and fills can use any color set in the graphics state, including patterns. PDF supports several types of patterns. The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly. This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn. Beginning with PDF 1.3 there is also a shading pattern, which draws continuously varying colors. There are seven types of shading patterns of which the simplest are the axial shading (Type 2) and radial shading (Type 3).

Raster images[edit]

Raster images in PDF (called Image XObjects) are represented by dictionaries with an associated stream. The dictionary describes the properties of the image, and the stream contains the image data. (Less commonly, small raster images may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes. Image filters supported in PDF include the following general-purpose filters:

• ASCII85Decode, a filter used to put the stream into 7-bit ASCII,
• ASCIIHexDecode, similar to ASCII85Decode but less compact,
• FlateDecode, a commonly used filter based on the deflate algorithm defined in RFC 1951 (deflate is also used in the gzip, PNG, and zip file formats among others); introduced in PDF 1.2; it can use one of two groups of predictor functions for more compact zlib/deflate compression: Predictor 2 from the TIFF 6.0 specification and predictors (filters) from the PNG specification (RFC 2083),
• LZWDecode, a filter based on LZW Compression; it can use one of two groups of predictor functions for more compact LZW compression: Predictor 2 from the TIFF 6.0 specification and predictors (filters) from the PNG specification,
• RunLengthDecode, a simple compression method for streams with repetitive data using the run-length encoding algorithm and the image-specific filters,
• DCTDecode, a lossy filter based on the JPEG standard,
• CCITTFaxDecode, a lossless bi-level (black/white) filter based on the Group 3 or Group 4 CCITT (ITU-T) fax compression standard defined in ITU-T T.4 and T.6,
• JBIG2Decode, a lossy or lossless bi-level (black/white) filter based on the JBIG2 standard, introduced in PDF 1.4, and
• JPXDecode, a lossy or lossless filter based on the JPEG 2000 standard, introduced in PDF 1.5.

Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of external streams or Alternate Images. Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these features.

Text[edit]

Text in PDF is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource.

A font object in PDF is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded font file. The latter case is called an embedded font while the former is called an unembedded font. The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType. Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.

Fourteen typefaces, known as the standard 14 fonts, have a special significance in PDF documents:

• Times (v3) (in regular, italic, bold, and bold italic)
• Courier (in regular, oblique, bold and bold oblique)
• Helvetica (v3) (in regular, oblique, bold and bold oblique)
• Symbol
• Zapf Dingbats

These fonts are sometimes called the base fourteen fonts.^[22] These fonts, or suitable substitute fonts with the same metrics, should be available in most PDF readers, but they are not guaranteed to be available in the reader, and may only display correctly if the system has them installed.^[23] Fonts may be substituted if they are not embedded in a PDF.

Within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding. There are several predefined encodings, including WinAnsi, MacRoman, and many encodings for East Asian languages and a font can have its own built-in encoding. (Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font’s built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts).^[2] The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.

For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for horizontal writing) and Identity-V (for vertical) are used. With such fonts, it is necessary to provide a ToUnicode table if semantic information about the characters is to be preserved.

Transparency[edit]

The original imaging model of PDF was, like PostScript’s, opaque: each object drawn on the page completely replaced anything previously marked in the same location. In PDF 1.4 the imaging model was extended to allow transparency. When transparency is used, new objects interact with previously marked objects to produce blending effects. The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to PDF 1.3 and earlier specifications. As a result, files that use a small amount of transparency might view acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer.

The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha. The model is closely aligned with the features of Adobe Illustrator version 9. The blend modes were based on those used by Adobe Photoshop at the time. When the PDF 1.4 specification was published, the formulas for calculating blend modes were kept secret by Adobe. They have since been published.^[24]

The concept of a transparency group in PDF specification is independent of existing notions of «group» or «layer» in applications such as Adobe Illustrator. Those groupings reflect logical relationships among objects that are meaningful when editing those objects, but they are not part of the imaging model.

Additional features[edit]

Logical structure and accessibility[edit]

A «tagged» PDF (see clause 14.8 in ISO 32000) includes document structure and semantics information to enable reliable text extraction and accessibility. Technically speaking, tagged PDF is a stylized use of the format that builds on the logical structure framework introduced in PDF 1.3. Tagged PDF defines a set of standard structure types and attributes that allow page content (text, graphics, and images) to be extracted and reused for other purposes.^[25]

Tagged PDF is not required in situations where a PDF file is intended only for print. Since the feature is optional, and since the rules for Tagged PDF were relatively vague in ISO 32000-1, support for tagged PDF among consuming devices, including assistive technology (AT), is uneven as of 2021.^[26] ISO 32000-2, however, includes an improved discussion of tagged PDF which is anticipated to facilitate further adoption.

An ISO-standardized subset of PDF specifically targeted at accessibility, PDF/UA, was first published in 2012.

Optional Content Groups (layers)[edit]

With the introduction of PDF version 1.5 (2003) came the concept of Layers. Layers, more formally known as Optional Content Groups (OCGs), refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or viewers. This capability is useful in CAD drawings, layered artwork, maps, multi-language documents, etc.

Basically, it consists of an Optional Content Properties Dictionary added to the document root. This dictionary contains an array of Optional Content Groups (OCGs), each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status (Displayed or Suppressed) of the given OCGs.

Encryption and signatures[edit]

A PDF file may be encrypted, for security, in which case a password is needed to view or edit the contents. PDF 2.0 defines 256-bit AES encryption as standard for PDF 2.0 files. The PDF Reference also defines ways that third parties can define their own encryption systems for PDF.

PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF is provided in ISO 32000-2.

PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing, or printing. These restrictions depend on the reader software to obey them, so the security they provide is limited.

The standard security provided by PDF consists of two different methods and two different passwords: a user password, which encrypts the file and prevents opening, and an owner password, which specifies operations that should be restricted even when the document is decrypted, which can include modifying, printing, or copying text and graphics out of the document, or adding or modifying text notes and AcroForm fields. The user password encrypts the file, while the owner password does not, instead relying on client software to respect these restrictions. An owner password can easily be removed by software, including some free online services.^[27] Thus, the use restrictions that a document author places on a PDF document are not secure, and cannot be assured once the file is distributed; this warning is displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.

Even without removing the password, most freeware or open source PDF readers ignore the permission «protections» and allow the user to print or make copy of excerpts of the text as if the document were not limited by password protection.^[28][29][30]

Beginning with PDF 1.5, Usage rights (UR) signatures are used to enable additional interactive features that are not available by default in a particular PDF viewer application. The signature is used to validate that the permissions have been granted by a bona fide granting authority. For example, it can be used to allow a user:^[31]

• To save the PDF document along with a modified form and/or annotation data
• Import form data files in FDF, XFDF, and text (CSV/TSV) formats
• Export form data files in FDF and XFDF formats
• Submit form data
• Instantiate new pages from named page templates
• Apply a digital signature to existing digital signature form field
• Create, delete, modify, copy, import, and export annotations

For example, Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. Adobe Reader verifies that the signature uses a certificate from an Adobe-authorized certificate authority. Any PDF application can use this same mechanism for its own purposes.^[31]

Under specific circumstances including non-patched systems of the receiver, the information the receiver of a digital signed document sees can be manipulated by the sender after the document has been signed by the signer.^[32]

PAdES (PDF Advanced Electronic Signatures) is a set of restrictions and extensions to PDF and ISO 32000-1^[33] making it suitable for advanced electronic signatures. This is published by ETSI as TS 102 778.^[34]

File attachments[edit]

PDF files can have file attachments which processors may access and open or save to a local filesystem.^[35]

Metadata[edit]

PDF files can contain two types of metadata.^[2] The first is the Document Information Dictionary, a set of key/value fields such as author, title, subject, creation and update dates. This is optional and is referenced from an Info key in the trailer of the file. A small set of fields is defined and can be extended with additional text values if required. This method is deprecated in PDF 2.0.

In PDF 1.4, support was added for Metadata Streams, using the Extensible Metadata Platform (XMP) to add XML standards-based extensible metadata as used in other file formats. PDF 2.0 allows metadata to be attached to any object in the document, such as information about embedded illustrations, fonts, and images, as well as the whole document (attaching to the document catalog), using an extensible schema.

PDF documents can also contain display settings, including the page display layout and zoom level in a Viewer Preferences object. Adobe Reader uses these settings to override the user’s default settings when opening the document.^[36] The free Adobe Reader cannot remove these settings.

Accessibility[edit]

PDF files can be created specifically to be accessible for people with disabilities.^{[37][38][39][40][41]} PDF file formats in use as of 2014 can include tags, text equivalents, captions, audio descriptions, and more. Some software can automatically produce tagged PDFs, but this feature is not always enabled by default.^[42][43] Leading screen readers, including JAWS, Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDF.^[44][45] Moreover, tagged PDFs can be re-flowed and magnified for readers with visual impairments. Adding tags to older PDFs and those that are generated from scanned documents can present some challenges.

One of the significant challenges with PDF accessibility is that PDF documents have three distinct views, which, depending on the document’s creation, can be inconsistent with each other. The three views are (i) the physical view, (ii) the tags view, and (iii) the content view. The physical view is displayed and printed (what most people consider a PDF document). The tags view is what screen readers and other assistive technologies use to deliver high-quality navigation and reading experience to users with disabilities. The content view is based on the physical order of objects within the PDF’s content stream and may be displayed by software that does not fully support the tags’ view, such as the Reflow feature in Adobe’s Reader.

PDF/UA, the International Standard for accessible PDF based on ISO 32000-1 was first published as ISO 14289–1 in 2012 and establishes normative language for accessible PDF technology.

Multimedia[edit]

Rich Media PDF is a PDF file including interactive content that can be embedded or linked within the file. It can contain images, audio, video content, or buttons. For example, if the interactive PDF is a digital catalog for an E-commerce business, products can be listed on the PDF pages and can be added with images and links to the website and buttons to order directly from the document.

Forms[edit]

Interactive Forms is a mechanism to add forms to the PDF file format. PDF currently supports two different methods for integrating data and PDF forms. Both formats today coexist in the PDF specification:^{[31][46][47][48]}

• AcroForms (also known as Acrobat forms), introduced in the PDF 1.2 format specification and included in all later PDF specifications.
• XML Forms Architecture (XFA) forms, introduced in the PDF 1.5 format specification. Adobe XFA Forms are not compatible with AcroForms.^[49] XFA was deprecated from PDF with PDF 2.0.

AcroForms were introduced in the PDF 1.2 format. AcroForms permit using objects (e.g. text boxes, Radio buttons, etc.) and some code (e.g. JavaScript). Alongside the standard PDF action types, interactive forms (AcroForms) support submitting, resetting, and importing data. The «submit» action transmits the names and values of selected interactive form fields to a specified uniform resource locator (URL). Interactive form field names and values may be submitted in any of the following formats, (depending on the settings of the action’s ExportFormat, SubmitPDF, and XFDF flags):^[31]

HTML Form format

HTML 4.01 Specification since PDF 1.5; HTML 2.0 since 1.2

Forms Data Format (FDF)

based on PDF, uses the same syntax and has essentially the same file structure, but is much simpler than PDF since the body of an FDF document consists of only one required object. Forms Data Format is defined in the PDF specification (since PDF 1.2). The Forms Data Format can be used when submitting form data to a server, receiving the response, and incorporating it into the interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. FDF was originally defined in 1996 as part of ISO 32000-2:2017.^{[citation needed]}

XML Forms Data Format (XFDF)

(external XML Forms Data Format Specification, Version 2.0; supported since PDF 1.5; it replaced the «XML» form submission format defined in PDF 1.4) the XML version of Forms Data Format, but the XFDF implements only a subset of FDF containing forms and annotations. Some entries in the FDF dictionary do not have XFDF equivalents – such as the Status, Encoding, JavaScript, Page’s keys, EmbeddedFDFs, Differences, and Target. In addition, XFDF does not allow the spawning, or addition, of new pages based on the given data; as can be done when using an FDF file. The XFDF specification is referenced (but not included) in PDF 1.5 specification (and in later versions). It is described separately in XML Forms Data Format Specification.^[50] The PDF 1.4 specification allowed form submissions in XML format, but this was replaced by submissions in XFDF format in the PDF 1.5 specification. XFDF conforms to the XML standard. XFDF can be used in the same way as FDF; e.g., form data is submitted to a server, modifications are made, then sent back and the new form data is imported in an interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. As of August 2019, XFDF 3.0 is an ISO/IEC standard under the formal name ISO 19444-1:2019 — Document management — XML Forms Data Format — Part 1: Use of ISO 32000-2 (XFDF 3.0).^[51] This standard is a normative reference of ISO 32000-2.

PDF

The entire document can be submitted rather than individual fields and values, as was defined in PDF 1.4.

AcroForms can keep form field values in external stand-alone files containing key-value pairs. The external files may use Forms Data Format (FDF) and XML Forms Data Format (XFDF) files.^[52][50][53] The usage rights (UR) signatures define rights for import form data files in FDF, XFDF, and text (CSV/TSV) formats, and export form data files in FDF and XFDF formats.^[31]

In PDF 1.5, Adobe Systems introduced a proprietary format for forms; Adobe XML Forms Architecture (XFA). Adobe XFA Forms are not compatible with ISO 32000’s AcroForms feature, and most PDF processors do not handle XFA content. The XFA specification is referenced from ISO 32000-1/PDF 1.7 as an external proprietary specification and was entirely deprecated from PDF with ISO 32000-2 (PDF 2.0).

Licensing[edit]

Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.^[54]

Security[edit]

In November 2019, researchers from Ruhr University Bochum and Hackmanit GmbH published attacks on digitally signed PDFs .^[55] They showed how to change the visible content in a signed PDF without invalidating the signature in 21 of 22 desktop PDF viewers and 6 of 8 online validation services by abusing implementation flaws.
At the same conference, they additionally showed how to exfiltrate the plaintext of encrypted content in PDFs.^[56] In 2021, they showed new so-called shadow attacks on PDFs that abuse the flexibility of features provided in the specification.^[57] An overview of security issues in PDFs regarding denial of service, information disclosure, data manipulation, and Arbitrary code execution attacks was presented by Jens Müller.^[58][59]

PDF attachments carrying viruses were first discovered in 2001. The virus, named OUTLOOK.PDFWorm or Peachy, uses Microsoft Outlook to send itself as an attached Adobe PDF file. It was activated with Adobe Acrobat, but not with Acrobat Reader.^[60]

From time to time, new vulnerabilities are discovered in various versions of Adobe Reader,^[61] prompting the company to issue security fixes. Other PDF readers are also susceptible. One aggravating factor is that a PDF reader can be configured to start automatically if a web page has an embedded PDF file, providing a vector for attack. If a malicious web page contains an infected PDF file that takes advantage of a vulnerability in the PDF reader, the system may be compromised even if the browser is secure. Some of these vulnerabilities are a result of the PDF standard allowing PDF documents to be scripted with JavaScript. Disabling JavaScript execution in the PDF reader can help mitigate such future exploits, although it does not protect against exploits in other parts of the PDF viewing software. Security experts say that JavaScript is not essential for a PDF reader and that the security benefit that comes from disabling JavaScript outweighs any compatibility issues caused.^[62] One way of avoiding PDF file exploits is to have a local or web service convert files to another format before viewing.

On March 30, 2010, security researcher Didier Stevens reported an Adobe Reader and Foxit Reader exploit that runs a malicious executable if the user allows it to launch when asked.^[63]

Software[edit]

Viewers and editors[edit]

PDF viewers are generally provided free of charge, and many versions are available from a variety of sources.

There are many software options for creating PDFs, including the PDF printing capabilities built into macOS, iOS,^[64] and most Linux distributions, LibreOffice, Microsoft Office 2007 (if updated to SP2) and later,^[65] WordPerfect 9, Scribus, numerous PDF print drivers for Microsoft Windows, the pdfTeX typesetting system, the DocBook PDF tools, applications developed around Ghostscript and Adobe Acrobat itself as well as Adobe InDesign, Adobe FrameMaker, Adobe Illustrator, Adobe Photoshop. Google’s online office suite Google Docs allows for uploading and saving to PDF. Some web apps offer free PDF editing and annotation tools.

The Free Software Foundation once thought of as one of their high priority projects to be «developing a free, high-quality and fully functional set of libraries and programs that implement the PDF file format and associated technologies to the ISO 32000 standard.»^[66][67] In 2011, however, the GNU PDF project was removed from the list of «high priority projects» due to the maturation of the Poppler library,^[68] which has enjoyed wider use in applications such as Evince with the GNOME desktop environment. Poppler is based on Xpdf^[69][70] code base. There are also commercial development libraries available as listed in List of PDF software.

The Apache PDFBox project of the Apache Software Foundation is an open source Java library for working with PDF documents. PDFBox is licensed under the Apache License.^[71]

Printing[edit]

Raster image processors (RIPs) are used to convert PDF files into a raster format suitable for imaging onto paper and other media in printers, digital production presses and prepress in a process known as rasterization. RIPs capable of processing PDF directly include the Adobe PDF Print Engine^[72] from Adobe Systems and Jaws^[73] and the Harlequin RIP from Global Graphics.

In 1993, the Jaws raster image processor from Global Graphics became the first shipping prepress RIP that interpreted PDF natively without conversion to another format. The company released an upgrade to their Harlequin RIP with the same capability in 1997.^[74]

Agfa-Gevaert introduced and shipped Apogee, the first prepress workflow system based on PDF, in 1997.

Many commercial offset printers have accepted the submission of press-ready PDF files as a print source, specifically the PDF/X-1a subset and variations of the same.^[75] The submission of press-ready PDF files is a replacement for the problematic need for receiving collected native working files.

In 2006, PDF was widely accepted as the standard print job format at the Open Source Development Labs Printing Summit. It is supported as a print job format by the Common Unix Printing System and desktop application projects such as GNOME, KDE, Firefox, Thunderbird, LibreOffice and OpenOffice have switched to emit print jobs in PDF.^[76]

Some desktop printers also support direct PDF printing, which can interpret PDF data without external help.

Native display model[edit]

PDF was selected as the «native» metafile format for Mac OS X, replacing the PICT format of the earlier classic Mac OS. The imaging model of the Quartz graphics layer is based on the model common to Display PostScript and PDF, leading to the nickname Display PDF. The Preview application can display PDF files, as can version 2.0 and later of the Safari web browser. System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the OS-standard printing architecture. The files are then exported in PDF 1.3 format according to the file header. When taking a screenshot under Mac OS X versions 10.0 through 10.3, the image was also captured as a PDF; later versions save screen captures as a PNG file, though this behavior can be set back to PDF if desired.

Annotation[edit]

Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, and add notes to already created PDF files. One UNIX application available as free software (under the GNU General Public License) is PDFedit. The freeware Foxit Reader, available for Microsoft Windows, macOS and Linux, allows annotating documents. Tracker Software’s PDF-XChange Viewer allows annotations and markups without restrictions in its freeware alternative. Apple’s macOS’s integrated PDF viewer, Preview, does also enable annotations as does the open-source software Skim, with the latter supporting interaction with LaTeX, SyncTeX, and PDFSync and integration with BibDesk reference management software. Freeware Qiqqa can create an annotation report that summarizes all the annotations and notes one has made across their library of PDFs. The Text Verification Tool exports differences in documents as annotations and markups.

There are also web annotation systems that support annotation in pdf and other documents formats. In cases where PDFs are expected to have all of the functionality of paper documents, ink annotation is required.

Alternatives[edit]

The Open XML Paper Specification is a competing format used both as a page description language and as the native print spooler format for Microsoft Windows since Windows Vista.

Mixed Object: Document Content Architecture is a competing format. MO:DCA-P is a part of Advanced Function Presentation.

See also[edit]

• Web document
• XSL Formatting Objects

References[edit]

Further reading[edit]

• Hardy, M. R. B.; Brailsford, D. F. (2002). «Mapping and displaying structural transformations between XML and PDF». Proceedings of the 2002 ACM symposium on Document engineering – DocEng ’02 (PDF). Proceedings of the 2002 ACM symposium on Document engineering. pp. 95–102. doi:10.1145/585058.585077. ISBN 1-58113-594-7. S2CID 9371237.^[relevant?]
• PDF 2.0 «ISO 32000-2:2020(en), Document management — Portable document format — Part 2: PDF 2.0». www.iso.org. Retrieved December 16, 2020.
• PDF 2.0 «ISO 32000-2:2017(en), Document management — Portable document format — Part 2: PDF 2.0». www.iso.org. Retrieved January 31, 2019.
• PDF 1.7 (ISO 32000-1:2008)
• PDF 1.7 and errata to 1.7 at the Wayback Machine (archived March 6, 2022)
• PDF 1.6 (ISBN 0-321-30474-8) and errata to 1.6 at the Wayback Machine (archived March 6, 2022)
• PDF 1.5 and errata to 1.5 at the Wayback Machine (archived December 22, 2021)
• PDF 1.4 (ISBN 0-201-75839-3) and errata to 1.4 at the Wayback Machine (archived March 6, 2022)
• PDF 1.3 (ISBN 0-201-61588-6) and errata to 1.3 at the Wayback Machine (archived March 6, 2022)
• PDF 1.2
• PDF 1.0 (ISBN 0-201-62628-4)

External links[edit]

• PDF Association – The PDF Association is the industry association for software developers producing or processing PDF files.
• Adobe PDF 101: Summary of PDF
• Adobe: PostScript vs. PDF – Official introductory comparison of PS, EPS vs. PDF.
• PDF Standards….transitioning the PDF specification from a de facto standard to a de jure standard at the Wayback Machine (archived April 24, 2011) – Information about PDF/E and PDF/UA specification for accessible documents file format (archived by The Wayback Machine)
• PDF/A-1 ISO standard published by the International Organization for Standardization (with corrigenda)
• PDF Reference and Adobe Extensions to the PDF Specification
• Portable Document Format: An Introduction for Programmers – Introduction to PDF vs. PostScript and PDF internals (up to v1.3)
• The Camelot Paper – the paper in which John Warnock outlined the project that created PDF
• Everything you wanted to know about PDF but was afraid to ask – recording of a talk by Leonard Rosenthol (45 mins) (Adobe Systems) at TUG 2007

Portable Document Format

Adobe PDF icon
Filename extension	.pdf
Internet media type	application/pdf,[1] application/x-pdf application/x-bzpdf application-gzpdf
Type code	PDF [1] (including a single space)
Uniform Type Identifier (UTI)	com.adobe.pdf
Magic number	%PDF
Developed by	Adobe Inc. (1991–2008) ISO (2008–)
Initial release	June 15, 1993; 29 years ago
Latest release	2.0
Extended to	PDF/A, PDF/E, PDF/UA, PDF/VT, PDF/X
Standard	ISO 32000-2
Open format?	Yes
Website	www.iso.org/standard/75839.html

Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems.^[2][3] Based on the PostScript language, each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, vector graphics, raster images and other information needed to display it. PDF has its roots in «The Camelot Project» initiated by Adobe co-founder John Warnock in 1991.^[4]

PDF was standardized as ISO 32000 in 2008.^[5] The last edition as ISO 32000-2:2020 was published in December 2020.

PDF files may contain a variety of content besides flat text and graphics including logical structuring elements, interactive elements such as annotations and form-fields, layers, rich media (including video content), three-dimensional objects using U3D or PRC, and various other data formats. The PDF specification also provides for encryption and digital signatures, file attachments, and metadata to enable workflows requiring these features.

History[edit]

Adobe Systems made the PDF specification available free of charge in 1993. In the early years PDF was popular mainly in desktop publishing workflows, and competed with a variety of formats such as DjVu, Envoy, Common Ground Digital Paper, Farallon Replica and even Adobe’s own PostScript format.

PDF was a proprietary format controlled by Adobe until it was released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO 32000-1:2008,^[6][7] at which time control of the specification passed to an ISO Committee of volunteer industry experts. In 2008, Adobe published a Public Patent License to ISO 32000-1 granting royalty-free rights for all patents owned by Adobe that are necessary to make, use, sell, and distribute PDF-compliant implementations.^[8]

PDF 1.7, the sixth edition of the PDF specification that became ISO 32000-1, includes some proprietary technologies defined only by Adobe, such as Adobe XML Forms Architecture (XFA) and JavaScript extension for Acrobat, which are referenced by ISO 32000-1 as normative and indispensable for the full implementation of the ISO 32000-1 specification.^[9] These proprietary technologies are not standardized and their specification is published only on Adobe’s website.^[10][11][12] Many of them are also not supported by popular third-party implementations of PDF.

In December 2020, the second edition of PDF 2.0, ISO 32000-2:2020, was published, including clarifications, corrections, and critical updates to normative references.^[13] ISO 32000-2 does not include any proprietary technologies as normative references.^[14]

Technical details[edit]

A PDF file is often a combination of vector graphics, text, and bitmap graphics. The basic types of content in a PDF are:

• Typeset text stored as content streams (i.e., not encoded in plain text);
• Vector graphics for illustrations and designs that consist of shapes and lines;
• Raster graphics for photographs and other types of images
• Multimedia objects in the document.

In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as a plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.

PDF combines three technologies:

• An equivalent subset of the PostScript page description programming language but in declarative form, for generating the layout and graphics.
• A font-embedding/replacement system to allow fonts to travel with the documents.
• A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.

PostScript language[edit]

PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. It can handle graphics and standard features of programming languages such as if statements and loop commands. PDF is largely based on PostScript but simplified to remove flow control features like these, while graphics commands equivalent to lineto remain.

Historically, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized.^{[clarification needed]} Any files, graphics, or fonts to which the document refers also are collected. Then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.^{[citation needed]}

As a document format, PDF has several advantages over PostScript:

• PDF contains tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
• PDF (since version 1.4) supports transparent graphics; PostScript does not.
• PostScript is an interpreted programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages in a PostScript document must be processed to determine the correct appearance of a given page, whereas each page in a PDF document is unaffected by the others. As a result, PDF viewers allow the user to quickly jump to the final pages of a long document, whereas a PostScript viewer needs to process all pages sequentially before being able to display the destination page (unless the optional PostScript Document Structuring Conventions have been carefully compiled and included).

PDF 1.6 and later supports interactive 3D documents embedded in a PDF file: 3D drawings can be embedded using U3D or PRC and various other data formats.^[15][16][17]

File format[edit]

A PDF file is organized using ASCII characters, except for certain elements that may have binary content.
The file starts with a header containing a magic number (as a readable string) and the version of the format, for example %PDF-1.7. The format is a subset of a COS («Carousel» Object Structure) format.^[18] A COS tree file consists primarily of objects, of which there are nine types:^[14]

• Boolean values, representing true or false
• Real numbers
• Integers
• Strings, enclosed within parentheses ((...)) or represented as hexadecimal within single angle brackets (<...>). Strings may contain 8-bit characters.
• Names, starting with a forward slash (/)
• Arrays, ordered collections of objects enclosed within square brackets ([...])
• Dictionaries, collections of objects indexed by names enclosed within double angle brackets (<<...>>)
• Streams, usually containing large amounts of optionally compressed binary data, preceded by a dictionary and enclosed between the stream and endstream keywords.
• The null object

Furthermore, there may be comments, introduced with the percent sign (%). Comments may contain 8-bit characters.

Objects may be either direct (embedded in another object) or indirect. Indirect objects are numbered with an object number and a generation number and defined between the obj and endobj keywords if residing in the document root. Beginning with PDF version 1.5, indirect objects (except other streams) may also be located in special streams known as object streams (marked /Type /ObjStm). This technique enables non-stream objects to have standard stream filters applied to them, reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF. Object streams do not support specifying an object’s generation number (other than 0).

An index table, also called the cross-reference table, is located near the end of the file and gives the byte offset of each indirect object from the start of the file.^[19] This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (incremental update). Before PDF version 1.5, the table would always be in a special ASCII format, be marked with the xref keyword, and follow the main body composed of indirect objects. Version 1.5 introduced optional cross-reference streams, which have the form of a standard stream object, possibly with filters applied. Such a stream may be used instead of the ASCII cross-reference table and contains the offsets and other information in binary format. The format is flexible in that it allows for integer width specification (using the /W array), so that for example, a document not exceeding 64 KiB in size may dedicate only 2  bytes for object offsets.

At the end of a PDF file is a footer containing

• The startxref keyword followed by an offset to the start of the cross-reference table (starting with the xref keyword) or the cross-reference stream object, followed by
• The %%EOF end-of-file marker.

If a cross-reference stream is not being used, the footer is preceded by the trailer keyword followed by a dictionary containing information that would otherwise be contained in the cross-reference stream object’s dictionary:

• A reference to the root object of the tree structure, also known as the catalog (/Root)
• The count of indirect objects in the cross-reference table (/Size)
• Other optional information

Within each page, there are one or multiple content streams that describe the text, vector and images being drawn on the page. The content stream is stack-based, similar to PostScript.^[20]

There are two layouts to the PDF files: non-linearized (not «optimized») and linearized («optimized»). Non-linearized PDF files can be smaller than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file. Linearized PDF files (also called «optimized» or «web optimized» PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since all objects required for the first page to display are optimally organized at the start of the file.^[21] PDF files may be optimized using Adobe Acrobat software or QPDF.

Imaging model[edit]

The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.

PDF graphics use a device-independent Cartesian coordinate system to describe the surface of a page. A PDF page description can use a matrix to scale, rotate, or skew graphical elements. A key concept in PDF is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description. PDF has (as of version 2.0) 25 graphics state properties, of which some of the most important are:

• The current transformation matrix (CTM), which determines the coordinate system
• The clipping path
• The color space
• The alpha constant, which is a key component of transparency
• Black point compensation control (introduced in PDF 2.0)

Vector graphics[edit]

As in PostScript, vector graphics in PDF are constructed with paths. Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text. Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves. Paths can be stroked, filled, fill then stroked, or used for clipping. Strokes and fills can use any color set in the graphics state, including patterns. PDF supports several types of patterns. The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly. This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn. Beginning with PDF 1.3 there is also a shading pattern, which draws continuously varying colors. There are seven types of shading patterns of which the simplest are the axial shading (Type 2) and radial shading (Type 3).

Raster images[edit]

Raster images in PDF (called Image XObjects) are represented by dictionaries with an associated stream. The dictionary describes the properties of the image, and the stream contains the image data. (Less commonly, small raster images may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes. Image filters supported in PDF include the following general-purpose filters:

• ASCII85Decode, a filter used to put the stream into 7-bit ASCII,
• ASCIIHexDecode, similar to ASCII85Decode but less compact,
• FlateDecode, a commonly used filter based on the deflate algorithm defined in RFC 1951 (deflate is also used in the gzip, PNG, and zip file formats among others); introduced in PDF 1.2; it can use one of two groups of predictor functions for more compact zlib/deflate compression: Predictor 2 from the TIFF 6.0 specification and predictors (filters) from the PNG specification (RFC 2083),
• LZWDecode, a filter based on LZW Compression; it can use one of two groups of predictor functions for more compact LZW compression: Predictor 2 from the TIFF 6.0 specification and predictors (filters) from the PNG specification,
• RunLengthDecode, a simple compression method for streams with repetitive data using the run-length encoding algorithm and the image-specific filters,
• DCTDecode, a lossy filter based on the JPEG standard,
• CCITTFaxDecode, a lossless bi-level (black/white) filter based on the Group 3 or Group 4 CCITT (ITU-T) fax compression standard defined in ITU-T T.4 and T.6,
• JBIG2Decode, a lossy or lossless bi-level (black/white) filter based on the JBIG2 standard, introduced in PDF 1.4, and
• JPXDecode, a lossy or lossless filter based on the JPEG 2000 standard, introduced in PDF 1.5.

Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of external streams or Alternate Images. Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these features.

Text[edit]

Text in PDF is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource.

A font object in PDF is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded font file. The latter case is called an embedded font while the former is called an unembedded font. The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType. Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.

Fourteen typefaces, known as the standard 14 fonts, have a special significance in PDF documents:

• Times (v3) (in regular, italic, bold, and bold italic)
• Courier (in regular, oblique, bold and bold oblique)
• Helvetica (v3) (in regular, oblique, bold and bold oblique)
• Symbol
• Zapf Dingbats

These fonts are sometimes called the base fourteen fonts.^[22] These fonts, or suitable substitute fonts with the same metrics, should be available in most PDF readers, but they are not guaranteed to be available in the reader, and may only display correctly if the system has them installed.^[23] Fonts may be substituted if they are not embedded in a PDF.

Within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding. There are several predefined encodings, including WinAnsi, MacRoman, and many encodings for East Asian languages and a font can have its own built-in encoding. (Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font’s built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts).^[2] The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.

For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for horizontal writing) and Identity-V (for vertical) are used. With such fonts, it is necessary to provide a ToUnicode table if semantic information about the characters is to be preserved.

Transparency[edit]

The original imaging model of PDF was, like PostScript’s, opaque: each object drawn on the page completely replaced anything previously marked in the same location. In PDF 1.4 the imaging model was extended to allow transparency. When transparency is used, new objects interact with previously marked objects to produce blending effects. The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to PDF 1.3 and earlier specifications. As a result, files that use a small amount of transparency might view acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer.

The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha. The model is closely aligned with the features of Adobe Illustrator version 9. The blend modes were based on those used by Adobe Photoshop at the time. When the PDF 1.4 specification was published, the formulas for calculating blend modes were kept secret by Adobe. They have since been published.^[24]

The concept of a transparency group in PDF specification is independent of existing notions of «group» or «layer» in applications such as Adobe Illustrator. Those groupings reflect logical relationships among objects that are meaningful when editing those objects, but they are not part of the imaging model.

Additional features[edit]

Logical structure and accessibility[edit]

A «tagged» PDF (see clause 14.8 in ISO 32000) includes document structure and semantics information to enable reliable text extraction and accessibility. Technically speaking, tagged PDF is a stylized use of the format that builds on the logical structure framework introduced in PDF 1.3. Tagged PDF defines a set of standard structure types and attributes that allow page content (text, graphics, and images) to be extracted and reused for other purposes.^[25]

Tagged PDF is not required in situations where a PDF file is intended only for print. Since the feature is optional, and since the rules for Tagged PDF were relatively vague in ISO 32000-1, support for tagged PDF among consuming devices, including assistive technology (AT), is uneven as of 2021.^[26] ISO 32000-2, however, includes an improved discussion of tagged PDF which is anticipated to facilitate further adoption.

An ISO-standardized subset of PDF specifically targeted at accessibility, PDF/UA, was first published in 2012.

Optional Content Groups (layers)[edit]

With the introduction of PDF version 1.5 (2003) came the concept of Layers. Layers, more formally known as Optional Content Groups (OCGs), refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or viewers. This capability is useful in CAD drawings, layered artwork, maps, multi-language documents, etc.

Basically, it consists of an Optional Content Properties Dictionary added to the document root. This dictionary contains an array of Optional Content Groups (OCGs), each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status (Displayed or Suppressed) of the given OCGs.

Encryption and signatures[edit]

A PDF file may be encrypted, for security, in which case a password is needed to view or edit the contents. PDF 2.0 defines 256-bit AES encryption as standard for PDF 2.0 files. The PDF Reference also defines ways that third parties can define their own encryption systems for PDF.

PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF is provided in ISO 32000-2.

PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing, or printing. These restrictions depend on the reader software to obey them, so the security they provide is limited.

The standard security provided by PDF consists of two different methods and two different passwords: a user password, which encrypts the file and prevents opening, and an owner password, which specifies operations that should be restricted even when the document is decrypted, which can include modifying, printing, or copying text and graphics out of the document, or adding or modifying text notes and AcroForm fields. The user password encrypts the file, while the owner password does not, instead relying on client software to respect these restrictions. An owner password can easily be removed by software, including some free online services.^[27] Thus, the use restrictions that a document author places on a PDF document are not secure, and cannot be assured once the file is distributed; this warning is displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.

Even without removing the password, most freeware or open source PDF readers ignore the permission «protections» and allow the user to print or make copy of excerpts of the text as if the document were not limited by password protection.^[28][29][30]

Beginning with PDF 1.5, Usage rights (UR) signatures are used to enable additional interactive features that are not available by default in a particular PDF viewer application. The signature is used to validate that the permissions have been granted by a bona fide granting authority. For example, it can be used to allow a user:^[31]

• To save the PDF document along with a modified form and/or annotation data
• Import form data files in FDF, XFDF, and text (CSV/TSV) formats
• Export form data files in FDF and XFDF formats
• Submit form data
• Instantiate new pages from named page templates
• Apply a digital signature to existing digital signature form field
• Create, delete, modify, copy, import, and export annotations

For example, Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. Adobe Reader verifies that the signature uses a certificate from an Adobe-authorized certificate authority. Any PDF application can use this same mechanism for its own purposes.^[31]

Under specific circumstances including non-patched systems of the receiver, the information the receiver of a digital signed document sees can be manipulated by the sender after the document has been signed by the signer.^[32]

PAdES (PDF Advanced Electronic Signatures) is a set of restrictions and extensions to PDF and ISO 32000-1^[33] making it suitable for advanced electronic signatures. This is published by ETSI as TS 102 778.^[34]

File attachments[edit]

PDF files can have file attachments which processors may access and open or save to a local filesystem.^[35]

Metadata[edit]

PDF files can contain two types of metadata.^[2] The first is the Document Information Dictionary, a set of key/value fields such as author, title, subject, creation and update dates. This is optional and is referenced from an Info key in the trailer of the file. A small set of fields is defined and can be extended with additional text values if required. This method is deprecated in PDF 2.0.

In PDF 1.4, support was added for Metadata Streams, using the Extensible Metadata Platform (XMP) to add XML standards-based extensible metadata as used in other file formats. PDF 2.0 allows metadata to be attached to any object in the document, such as information about embedded illustrations, fonts, and images, as well as the whole document (attaching to the document catalog), using an extensible schema.

PDF documents can also contain display settings, including the page display layout and zoom level in a Viewer Preferences object. Adobe Reader uses these settings to override the user’s default settings when opening the document.^[36] The free Adobe Reader cannot remove these settings.

Accessibility[edit]

PDF files can be created specifically to be accessible for people with disabilities.^{[37][38][39][40][41]} PDF file formats in use as of 2014 can include tags, text equivalents, captions, audio descriptions, and more. Some software can automatically produce tagged PDFs, but this feature is not always enabled by default.^[42][43] Leading screen readers, including JAWS, Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDF.^[44][45] Moreover, tagged PDFs can be re-flowed and magnified for readers with visual impairments. Adding tags to older PDFs and those that are generated from scanned documents can present some challenges.

One of the significant challenges with PDF accessibility is that PDF documents have three distinct views, which, depending on the document’s creation, can be inconsistent with each other. The three views are (i) the physical view, (ii) the tags view, and (iii) the content view. The physical view is displayed and printed (what most people consider a PDF document). The tags view is what screen readers and other assistive technologies use to deliver high-quality navigation and reading experience to users with disabilities. The content view is based on the physical order of objects within the PDF’s content stream and may be displayed by software that does not fully support the tags’ view, such as the Reflow feature in Adobe’s Reader.

PDF/UA, the International Standard for accessible PDF based on ISO 32000-1 was first published as ISO 14289–1 in 2012 and establishes normative language for accessible PDF technology.

Multimedia[edit]

Rich Media PDF is a PDF file including interactive content that can be embedded or linked within the file. It can contain images, audio, video content, or buttons. For example, if the interactive PDF is a digital catalog for an E-commerce business, products can be listed on the PDF pages and can be added with images and links to the website and buttons to order directly from the document.

Forms[edit]

Interactive Forms is a mechanism to add forms to the PDF file format. PDF currently supports two different methods for integrating data and PDF forms. Both formats today coexist in the PDF specification:^{[31][46][47][48]}

• AcroForms (also known as Acrobat forms), introduced in the PDF 1.2 format specification and included in all later PDF specifications.
• XML Forms Architecture (XFA) forms, introduced in the PDF 1.5 format specification. Adobe XFA Forms are not compatible with AcroForms.^[49] XFA was deprecated from PDF with PDF 2.0.

AcroForms were introduced in the PDF 1.2 format. AcroForms permit using objects (e.g. text boxes, Radio buttons, etc.) and some code (e.g. JavaScript). Alongside the standard PDF action types, interactive forms (AcroForms) support submitting, resetting, and importing data. The «submit» action transmits the names and values of selected interactive form fields to a specified uniform resource locator (URL). Interactive form field names and values may be submitted in any of the following formats, (depending on the settings of the action’s ExportFormat, SubmitPDF, and XFDF flags):^[31]

HTML Form format

HTML 4.01 Specification since PDF 1.5; HTML 2.0 since 1.2

Forms Data Format (FDF)

based on PDF, uses the same syntax and has essentially the same file structure, but is much simpler than PDF since the body of an FDF document consists of only one required object. Forms Data Format is defined in the PDF specification (since PDF 1.2). The Forms Data Format can be used when submitting form data to a server, receiving the response, and incorporating it into the interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. FDF was originally defined in 1996 as part of ISO 32000-2:2017.^{[citation needed]}

XML Forms Data Format (XFDF)

(external XML Forms Data Format Specification, Version 2.0; supported since PDF 1.5; it replaced the «XML» form submission format defined in PDF 1.4) the XML version of Forms Data Format, but the XFDF implements only a subset of FDF containing forms and annotations. Some entries in the FDF dictionary do not have XFDF equivalents – such as the Status, Encoding, JavaScript, Page’s keys, EmbeddedFDFs, Differences, and Target. In addition, XFDF does not allow the spawning, or addition, of new pages based on the given data; as can be done when using an FDF file. The XFDF specification is referenced (but not included) in PDF 1.5 specification (and in later versions). It is described separately in XML Forms Data Format Specification.^[50] The PDF 1.4 specification allowed form submissions in XML format, but this was replaced by submissions in XFDF format in the PDF 1.5 specification. XFDF conforms to the XML standard. XFDF can be used in the same way as FDF; e.g., form data is submitted to a server, modifications are made, then sent back and the new form data is imported in an interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. As of August 2019, XFDF 3.0 is an ISO/IEC standard under the formal name ISO 19444-1:2019 — Document management — XML Forms Data Format — Part 1: Use of ISO 32000-2 (XFDF 3.0).^[51] This standard is a normative reference of ISO 32000-2.

PDF

The entire document can be submitted rather than individual fields and values, as was defined in PDF 1.4.

AcroForms can keep form field values in external stand-alone files containing key-value pairs. The external files may use Forms Data Format (FDF) and XML Forms Data Format (XFDF) files.^[52][50][53] The usage rights (UR) signatures define rights for import form data files in FDF, XFDF, and text (CSV/TSV) formats, and export form data files in FDF and XFDF formats.^[31]

In PDF 1.5, Adobe Systems introduced a proprietary format for forms; Adobe XML Forms Architecture (XFA). Adobe XFA Forms are not compatible with ISO 32000’s AcroForms feature, and most PDF processors do not handle XFA content. The XFA specification is referenced from ISO 32000-1/PDF 1.7 as an external proprietary specification and was entirely deprecated from PDF with ISO 32000-2 (PDF 2.0).

Licensing[edit]

Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.^[54]

Security[edit]

In November 2019, researchers from Ruhr University Bochum and Hackmanit GmbH published attacks on digitally signed PDFs .^[55] They showed how to change the visible content in a signed PDF without invalidating the signature in 21 of 22 desktop PDF viewers and 6 of 8 online validation services by abusing implementation flaws.
At the same conference, they additionally showed how to exfiltrate the plaintext of encrypted content in PDFs.^[56] In 2021, they showed new so-called shadow attacks on PDFs that abuse the flexibility of features provided in the specification.^[57] An overview of security issues in PDFs regarding denial of service, information disclosure, data manipulation, and Arbitrary code execution attacks was presented by Jens Müller.^[58][59]

PDF attachments carrying viruses were first discovered in 2001. The virus, named OUTLOOK.PDFWorm or Peachy, uses Microsoft Outlook to send itself as an attached Adobe PDF file. It was activated with Adobe Acrobat, but not with Acrobat Reader.^[60]

From time to time, new vulnerabilities are discovered in various versions of Adobe Reader,^[61] prompting the company to issue security fixes. Other PDF readers are also susceptible. One aggravating factor is that a PDF reader can be configured to start automatically if a web page has an embedded PDF file, providing a vector for attack. If a malicious web page contains an infected PDF file that takes advantage of a vulnerability in the PDF reader, the system may be compromised even if the browser is secure. Some of these vulnerabilities are a result of the PDF standard allowing PDF documents to be scripted with JavaScript. Disabling JavaScript execution in the PDF reader can help mitigate such future exploits, although it does not protect against exploits in other parts of the PDF viewing software. Security experts say that JavaScript is not essential for a PDF reader and that the security benefit that comes from disabling JavaScript outweighs any compatibility issues caused.^[62] One way of avoiding PDF file exploits is to have a local or web service convert files to another format before viewing.

On March 30, 2010, security researcher Didier Stevens reported an Adobe Reader and Foxit Reader exploit that runs a malicious executable if the user allows it to launch when asked.^[63]

Software[edit]

Viewers and editors[edit]

PDF viewers are generally provided free of charge, and many versions are available from a variety of sources.

There are many software options for creating PDFs, including the PDF printing capabilities built into macOS, iOS,^[64] and most Linux distributions, LibreOffice, Microsoft Office 2007 (if updated to SP2) and later,^[65] WordPerfect 9, Scribus, numerous PDF print drivers for Microsoft Windows, the pdfTeX typesetting system, the DocBook PDF tools, applications developed around Ghostscript and Adobe Acrobat itself as well as Adobe InDesign, Adobe FrameMaker, Adobe Illustrator, Adobe Photoshop. Google’s online office suite Google Docs allows for uploading and saving to PDF. Some web apps offer free PDF editing and annotation tools.

The Free Software Foundation once thought of as one of their high priority projects to be «developing a free, high-quality and fully functional set of libraries and programs that implement the PDF file format and associated technologies to the ISO 32000 standard.»^[66][67] In 2011, however, the GNU PDF project was removed from the list of «high priority projects» due to the maturation of the Poppler library,^[68] which has enjoyed wider use in applications such as Evince with the GNOME desktop environment. Poppler is based on Xpdf^[69][70] code base. There are also commercial development libraries available as listed in List of PDF software.

The Apache PDFBox project of the Apache Software Foundation is an open source Java library for working with PDF documents. PDFBox is licensed under the Apache License.^[71]

Printing[edit]

Raster image processors (RIPs) are used to convert PDF files into a raster format suitable for imaging onto paper and other media in printers, digital production presses and prepress in a process known as rasterization. RIPs capable of processing PDF directly include the Adobe PDF Print Engine^[72] from Adobe Systems and Jaws^[73] and the Harlequin RIP from Global Graphics.

In 1993, the Jaws raster image processor from Global Graphics became the first shipping prepress RIP that interpreted PDF natively without conversion to another format. The company released an upgrade to their Harlequin RIP with the same capability in 1997.^[74]

Agfa-Gevaert introduced and shipped Apogee, the first prepress workflow system based on PDF, in 1997.

Many commercial offset printers have accepted the submission of press-ready PDF files as a print source, specifically the PDF/X-1a subset and variations of the same.^[75] The submission of press-ready PDF files is a replacement for the problematic need for receiving collected native working files.

In 2006, PDF was widely accepted as the standard print job format at the Open Source Development Labs Printing Summit. It is supported as a print job format by the Common Unix Printing System and desktop application projects such as GNOME, KDE, Firefox, Thunderbird, LibreOffice and OpenOffice have switched to emit print jobs in PDF.^[76]

Some desktop printers also support direct PDF printing, which can interpret PDF data without external help.

Native display model[edit]

PDF was selected as the «native» metafile format for Mac OS X, replacing the PICT format of the earlier classic Mac OS. The imaging model of the Quartz graphics layer is based on the model common to Display PostScript and PDF, leading to the nickname Display PDF. The Preview application can display PDF files, as can version 2.0 and later of the Safari web browser. System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the OS-standard printing architecture. The files are then exported in PDF 1.3 format according to the file header. When taking a screenshot under Mac OS X versions 10.0 through 10.3, the image was also captured as a PDF; later versions save screen captures as a PNG file, though this behavior can be set back to PDF if desired.

Annotation[edit]

Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, and add notes to already created PDF files. One UNIX application available as free software (under the GNU General Public License) is PDFedit. The freeware Foxit Reader, available for Microsoft Windows, macOS and Linux, allows annotating documents. Tracker Software’s PDF-XChange Viewer allows annotations and markups without restrictions in its freeware alternative. Apple’s macOS’s integrated PDF viewer, Preview, does also enable annotations as does the open-source software Skim, with the latter supporting interaction with LaTeX, SyncTeX, and PDFSync and integration with BibDesk reference management software. Freeware Qiqqa can create an annotation report that summarizes all the annotations and notes one has made across their library of PDFs. The Text Verification Tool exports differences in documents as annotations and markups.

There are also web annotation systems that support annotation in pdf and other documents formats. In cases where PDFs are expected to have all of the functionality of paper documents, ink annotation is required.

Alternatives[edit]

The Open XML Paper Specification is a competing format used both as a page description language and as the native print spooler format for Microsoft Windows since Windows Vista.

Mixed Object: Document Content Architecture is a competing format. MO:DCA-P is a part of Advanced Function Presentation.

See also[edit]

• Web document
• XSL Formatting Objects

References[edit]

Further reading[edit]

• Hardy, M. R. B.; Brailsford, D. F. (2002). «Mapping and displaying structural transformations between XML and PDF». Proceedings of the 2002 ACM symposium on Document engineering – DocEng ’02 (PDF). Proceedings of the 2002 ACM symposium on Document engineering. pp. 95–102. doi:10.1145/585058.585077. ISBN 1-58113-594-7. S2CID 9371237.^[relevant?]
• PDF 2.0 «ISO 32000-2:2020(en), Document management — Portable document format — Part 2: PDF 2.0». www.iso.org. Retrieved December 16, 2020.
• PDF 2.0 «ISO 32000-2:2017(en), Document management — Portable document format — Part 2: PDF 2.0». www.iso.org. Retrieved January 31, 2019.
• PDF 1.7 (ISO 32000-1:2008)
• PDF 1.7 and errata to 1.7 at the Wayback Machine (archived March 6, 2022)
• PDF 1.6 (ISBN 0-321-30474-8) and errata to 1.6 at the Wayback Machine (archived March 6, 2022)
• PDF 1.5 and errata to 1.5 at the Wayback Machine (archived December 22, 2021)
• PDF 1.4 (ISBN 0-201-75839-3) and errata to 1.4 at the Wayback Machine (archived March 6, 2022)
• PDF 1.3 (ISBN 0-201-61588-6) and errata to 1.3 at the Wayback Machine (archived March 6, 2022)
• PDF 1.2
• PDF 1.0 (ISBN 0-201-62628-4)

External links[edit]

• PDF Association – The PDF Association is the industry association for software developers producing or processing PDF files.
• Adobe PDF 101: Summary of PDF
• Adobe: PostScript vs. PDF – Official introductory comparison of PS, EPS vs. PDF.
• PDF Standards….transitioning the PDF specification from a de facto standard to a de jure standard at the Wayback Machine (archived April 24, 2011) – Information about PDF/E and PDF/UA specification for accessible documents file format (archived by The Wayback Machine)
• PDF/A-1 ISO standard published by the International Organization for Standardization (with corrigenda)
• PDF Reference and Adobe Extensions to the PDF Specification
• Portable Document Format: An Introduction for Programmers – Introduction to PDF vs. PostScript and PDF internals (up to v1.3)
• The Camelot Paper – the paper in which John Warnock outlined the project that created PDF
• Everything you wanted to know about PDF but was afraid to ask – recording of a talk by Leonard Rosenthol (45 mins) (Adobe Systems) at TUG 2007

PDF

PDF

Расширение файла:	.pdf
Тип MIME:	application/pdf
Type code:	‘PDF ‘ (включая пробел)
Разработчик:	Adobe Systems
Тип формата:	Графические форматы

PDF (аббревиатура от англ. Portable Document Format — переносимый формат документов; правильно произносить пи-ди-э́ф, но большее распространенние среди русскоязычных компьютерных специалистов получило произношение пэ-дэ-э́ф) — кроссплатформенный формат электронных документов, созданный фирмой Adobe Systems с использованием ряда возможностей языка PostScript. В первую очередь предназначен для представления в электронном виде полиграфической продукции, — значительное количество современного профессионального печатного оборудования может обрабатывать PDF непосредственно. Для просмотра можно использовать официальную бесплатную программу Adobe Reader, а также программы сторонних разработчиков. Традиционным способом создания PDF-документов является виртуальный принтер, то есть документ как таковой готовится в своей специализированной программе — графическом или текстовом редакторе, САПР и т. д., а затем экспортируется в формат PDF для распространения в электронном виде, передачи в типографию и т. п.

PDF с 1 июля 2008 года является открытым стандартом ISO 32000.^[1][2]

Формат PDF позволяет внедрять необходимые шрифты (построчный текст), векторные и растровые изображения, формы и мультимедиа-вставки. Поддерживает RGB, CMYK, Grayscale, Lab, Duotone, Bitmap, несколько типов сжатия растровой информации. Имеет собственные технические форматы для полиграфии: PDF/X-1, PDF/X-3. Включает механизм электронных подписей для защиты и проверки подлинности документов. В этом формате распространяется большое количество сопутствующей документации.

Версии

Существует несколько спецификаций pdf-документов последовательно расширяющих друг-друга. Для каждой новой спецификации создается новая версия Adobe Reader и Adobe Acrobat. Ниже показана таблица соответствий версий документов и версий программ в которых впервые была введена поддержка этих документов. Версию любого pdf-документа можно узнать по первым восьми байтам, открыв этот документ в текстовом режиме, например, в блокноте.

год	версия документа	новые возможности	версия ПО
1993	PDF 1.0		Acrobat 1.0
1994	PDF 1.1	пароли, ссылки, потоки, независимая от устройства цветопередача	Acrobat 2.0
1996	PDF 1.2	интерактивные элементы, обработка событий мыши, мультимедийные типы, уникод, улучшенное представление цвета и графики	Acrobat 3.0
1999	PDF 1.3	цифровые подписи, цветовые пространства ICC и DeviceN, JavaScript	Acrobat 4.0
2001	PDF 1.4	JBIG2, прозрачность, текстовый слой OCR	Acrobat 5.0
2003	PDF 1.5	JPEG 2000, связанное мультимедиа, объектные потоки, перекрестные потоки	Acrobat 6.0
2005	PDF 1.6	внедренное мультимедиа, XML-формы, AES-шифрование	Acrobat 7.0
2006	PDF 1.7		Acrobat 8.0
2008	PDF 1.7, AEL3	AES-шифрование 256-битным ключом	Acrobat 9.0
2009	PDF 1.7, AEL5	XFA 3.0	Acrobat 9.1

Сторонние программы для работы с PDF

Кроссплатформенные

• OpenOffice.org — свободный кроссплатформенный офисный пакет, имеющий функцию экспорта в PDF.
• pdftex (англ.)/pdflatex — вариант системы компьютерной вёрстки TeX/LaTeX, напрямую создающий PDF-файлы.
• Ghostscript (англ.) — свободный программый интерпретатор языка PostScript. Может использоваться для создания, преобразования и просмотра PDF-файлов.
• Xpdf (англ.) — свободная программа просмотра PDF-файлов для X Window System. Используется как движок для многих других программ просмотра.

Unix-подобные

• Okular — универсальное приложение для просмотра документов; часть KDE4.
• Evince — свободная программа для просмотра PDF, PostScript и других похожих форматов; часть GNOME
• KPDF — программа просмотра PDF-файлов для KDE (в KDE4 упразднена в пользу Okular).
• XPDF — программа просмотра PDF-файлов. Без привязки к библиотекам QT и GTK.
• epdfview — свободная программа просмотра PDF на библиотеке GTK, но без использования библиотек среды GNOME

Microsoft Windows

• Microsoft Office 2007 — в пакет обновлений SP2 встроена функция экспорта любых документов в PDF.
• Foxit Reader — условно-бесплатная программа для просмотра PDF-файлов в Microsoft Windows. Объём программы составляет 3,5 Мб, установки не требует.
• Sumatra PDF — свободная (GPLv2) программа для просмотра PDF-файлов в Microsoft Windows.
• ABBYY PDF Transformer — собственническая shareware программа под Windows NT от 5.0 для создания и преобразования PDF-файлов из любого офисного приложения и преобразование PDF-файлов в документы редактируемых форматов (Microsoft Word, RTF и др.).
• PDFCreator — свободная программа для создания файлов PDF. Может использоваться с любым приложением Microsoft Windows, обладающим возможностью печати документов.
• Scientific and technical documentation utility: STDU Viewer — бесплатная для некоммерческого использования программа для чтения PDF и DJVU файлов; STDU Converter — платная программа для преобразования DJVU в PDF.

Онлайновые

• Scribd

Примечания

См. также

• DjVu
• PDF/A
• XPS

Ссылки

• PDF Specifications, including the PDF Reference for PDF 1.7, PDF 1.6 (ISBN 0-321-30474-8), PDF 1.5, PDF 1.4 (ISBN 0-201-75839-3), PDF 1.3 (ISBN 0-201-61588-6)
• Adobe PDF 101: Quick overview of PDF — pdf-документ с описанием основных возможностей формата.

Графические форматы
Статичные Анимационные Растровые BMP • DjVu • GIF • HD Photo • ICO • ILBM • JBIG • JBIG2 • JPEG • JPEG 2000 • JPEG-LS • OpenEXR • PCX • PNG • PSD • RAW • TGA • TIFF • WBMP APNG • GIF • MNG Векторные AI • EPS • PDF • PostScript • SVG • WMF SVG • SWF

Всего найдено: 40

Здравствуйте! Всегда занимал вопрос, почему корни мак-мок и равн-ровн считаются корнями с чередующейся гласной, а не разными корнями. Ведь смысл у слов с этими корнями разный… Мы ведь не считаем одним корнем корни в словах развеваться и развиваться… Мне кажется, это очень путает при изучении языка и, особенно, при сдаче экзаменов. Я училась в 70-е годы прошлого века, мы учили только те корни, где, действительно, были трудности написания и где было четкое правило (исключения, конечно, учили тоже). Сейчас вопрос в ЕГЭ на эту тему можно вполне назвать «вопрос с подставой», хотя особых трудностей в написании здесь нет…

Ответ справочной службы русского языка

Корни в парах равн-ровн и мак-мок исторически восходят к одному корню, значения их дифференцировались, однако процесс этот не завершен. Есть слова-исключения и слова, в которых совмещается значение, свойственное обоим корням. Подробнее на Ваш вопрос отвечает статья Е. В. Арутюновой, Е. В. Бешенковой, О. Е. Ивановой «pdf» target=»_blank» rel=»noopener noreferrer»>Корень с чередованием равн-/ровн- (из материалов академического описания русской орфографии)» (журнал «Русский язык в школе». 2020. Т. 81. № 6. С. 90–96).

Кто из русских императоров принимал участие в создании русского гражданского шрифта?

Ответ справочной службы русского языка

Это был император Петр Великий. Подробнее о создании гражданской азбуки можно прочитать в терминологическом словаре С. В. Друговейко-Должанской и М. Б. Попова «pdf» target=»_blank» rel=»noopener noreferrer»>Современное русское письмо: графика, орфография, пунктуация».

Добрый день! Учитель в тетради по математике ребенку везде, где встречалось обозначение скорости км/ч или м/мин после ч и мин поставила точки. Хотя в учебнике их нет и поставила за это три. я прочитала везде, что в этих сокращениях точка не ставится, если сокращение общепринятое. Скажите, пожалуйста, как правильно и на что можно сослаться в диалоге с учителем? Спасибо, Алена

Ответ справочной службы русского языка

Сослаться можно на «Правила русской орфографии и пунктуации. Полный академический справочник» (под ред. В. В. Лопатина. М., 2006 и др. издания, § 209), на ГОСТ 8.417-2002 «pdf» target=»_blank» rel=»noopener noreferrer»>Межгосударственный стандарт. Государственная система обеспечения единства измерений. Единицы величин» (введен в действие Постановлением Госстандарта РФ от 04.02.2003 № 38-ст).

Уважаемые, всем привет! Желается получить от вас грамотный ответ на один спорный вопрос. Предыстория такова, что есть некая сеть, где общаются люди, и есть правила общения. Правило а) звучит так — штраф запрещён если ваш ник (псевдоним) не упомянут в одной строчке с оскорбительными словами А правило б) так — штраф запрещён если вы находитесь в диалоге с тем, кому желаете выписать штраф, пусть даже этот диалог непрямой — я хочу убедить владельца этого канала на той сети в том, что ДИАЛОГ бывает ТОЛЬКО прямой, и не подразумевает мифических разночтений как выше — «непрямой», «косвенный» диалог. Прошу подтвердить мою правоту по возможности как можно более официальным ответом, pdf‘ом с печатью, или что-то типа того :)) Благодарю заранее ! -Виктор.

Ответ справочной службы русского языка

Верно ли мы понимаем вопрос: можно ли назвать диалогом беседу, участники которой отвечают на реплики других участников не им лично, а в репликах, адресованных другим участникам беседы? Полагаем, что такую беседу также можно называть диалогом.

Добрый день! Подскажите, пожалуйста, каким ГОСТом сегодня (2019 год) следует руководствоваться при оформлении библиографического списка к научной статье для публикации в журнале (в списке ВАК)?

Ответ справочной службы русского языка

См. pdf»>здесь.

Дополнение к вопросу № 301206 …как правильно произносить английское слово PDF (формат электронных документов) на русском языке. Ответ справочной службы русского языка: Наиболее корректно произношение в соответствии с названиями букв английского алфавита [пидиэф], однако наиболее распространенный вариант произношения — [пэдээф]. Я всегда говорил PDF [пидиэф]. Не слышал иного произношения.

Ответ справочной службы русского языка

Удивительно. Возможно, дело в круге общения или местности.

Добрый день! Подскажите, пожалуйста, как правильно произносить английское слово PDF (формат электронных документов) на русском языке. Каковы правила чтения иностранных аббревиатур? Заранее благодарю!

Ответ справочной службы русского языка

Наиболее корректно произношение в соответствии с названиями букв английского алфавита [пидиэф], однако наиболее распространенный вариант произношения — [пэдээф].

Добрый день! У меня немного необычный лингвистический вопрос, возникший при написании статей. Возможно, вы сможете что-то подсказать. Если девушка, например, Ольга Ивановна Годунова, использует псевдоним, например, Саурон или Гэндальф — мужское имя, то каким образом в статье должно склоняться имя? «Саурон сделала» или «Саурон сделал»? Не в рамках описания роли на сцене театра, а просто как избранный псевдоним в повседневной жизни, но в том же сообществе, то есть через строку идет упоминание женского имени. Например: «Саурон сделал(а) доклад». И: «Ольга Ивановна Годунова также утверждает в нем, …» . Другой аналогичный вариант текста в статье, с непосредственным уточнением в скобках: «Саурон (Ольга Ивановна Годунова) сделал(а) доклад». И далее по тексту. Есть ли какие-то законы или правила, регулирующие склонение и семантическое согласование в таких случаях?

Ответ справочной службы русского языка

Правила об употреблении подобных псевдонимов нет, но можно попробовать опереться на прецедент. Псевдоним Жорж Санд устойчиво используется как женское имя. Например: 

Отчего, например, так понравилась Жорж Санд? «А потому, ― отвечает Белинский, ― что для нее не существуют ни аристократы, ни плебеи; для нее существует только человек, и она находит человека во всех сословиях, во всех слоях общества, любит его, сострадает ему, гордится им и плачет за него…» [Е. А. Соловьев-Андреевич. Александр Герцен. Его жизнь и литературная деятельность (1897)] 

Жорж Санд стала известной сразу по выходу первых романов ― «Индиана» и «Валентина». [А. Всеволжский. Мятежная Аврора (2004) // «Вокруг света», 2004.07.15]

Но один из пятидесяти экземпляров, отпечатанных в 1846 году, приобрела Е. Г. Бекетова, переводчица Вальтера Скотта, Диккенса, Теккерея, Жорж Санд, Гюго, Бальзака и многих других прекрасных писателей. [В. Баевский. Ассиар // «Знамя», 2005]    

Однако интересно, что современник писательницы Н. Г. Чернышевский в статье «pdf» target=»_blank» rel=»noopener noreferrer»>Жизнь Жоржа Санда», комментируя публикацию ее мемуаров, склоняет новаторский для того времени псевдоним Джорж Санд как мужское имя, а согласует с ним слова как с женским именем. Ср.:

Едва ли какое-нибудь явление в изящной словесности последних двух или трех лет имело столь сильный успех, как мемуары Жоржа Санда. <…> Казалось, что Жорж Санд намерена дать им [«Записок»] громадный размер… <…> Итак, вполне переводить «Записки» г-жи Жорж Санд невозможно. Тем не менее, автобиография заключает в себе очень много интересных фактов и прекрасных эпизодов. Иначе и быть не могло. Жизнь Жоржа Санда замечательна не только высоким психологическим развитием: она также богата драматическими [положениями]. Все это вместе сообщает ее «Запискам» высокую занимательность. Кроме того, Жорж Санд принимала сильное участие в исторических событиях, сближалась со многими замечательными людьми, и ее «Записки» прекрасно знакомят нас с некоторыми из них. <…> 

Вероятно, в этой статье отражен начальный этап освоения мужского имени как женского псевдонима. Сейчас для нас более естественными кажутся сочетания жизнь Жорж Санд, мемуары Жорж Санд.

Здравствуйте! Подскажите, пожалуйста, лексическое значение слов в толковом словаре на сайте проверяется? Из какого источника берут эти значения. И с какого момента сниженная разговорная лексика является нормой для объяснения лексического значения? Этот вопрос возник после обращения к вашему сайту для уточнения лексического значения слова «кровать». Очень сметило слово «спанье». Заранее благодарю за ответ.

Ответ справочной службы русского языка

Большой толковый словарь, которым Вы пользуетесь на портале, – это авторская редакция словаря, вышедшего под грифом Института лингвистических исследований РАН. Подробнее см. здесь. 

Слово спанье, которое использовано в словарной статье «кровать», разговорное, но разговорная лексика является одной из лексических подсистем литературного языка. И толкование слова кровать с использованием существительного спанье можно назвать лексикографической традицией. Такое толкование можно найти в «Толковом словаре русского языка» под ред. Д. Н. Ушакова (1935-1940), в словарях С. И. Ожегова и Н. Ю. Шведовой, в «pdf/05.pdf» target=»_blank» rel=»noopener noreferrer»>Большом академическом словаре». 

Скажите, пожалуйста! Как правильно — вдаль или в даль?

Ответ справочной службы русского языка

Возможно слитное и раздельное написание. Слитно пишется наречие вдаль (глядеть вдаль), раздельно – сочетание предлога с существительным (в даль моря, в даль веков, в даль прожитых лет, всматриваться в даль). 

Разграничить случаи слитного и раздельного написания иногда очень сложно. Подробнее см. в статье Н. Б. Кошкарёвой «Куда уходит дорога? И куда уходит жизнь? (о слитном / раздельном написании вдаль и в даль)» во втором выпуске pdf» target=»_blank» rel=»noopener noreferrer»>сборника научных трудов по материалам Тотального диктанта. 

Здравствуйте! Подскажите, как правильно писать сочетания иностранной аббревиатуры на латинице и слова на русском. Например pdf*файл и png*изображение. Нужен дефис или нет? Спасибо.

Ответ справочной службы русского языка

Корректно: pdf-файл, png-изображение.

Здравствуйте, редакция портала «Грамота.ру»! Скажите, пожалуйста, какой частью речи является слово «ограничеН/ННо» в предложении «Кроме того, количество модификаций, которые можно установить одновременно, ограничеН/ННо» и, следовательно, как нужно это слово писать. Каким правилом нужно руководствоваться при выборе раздельного/дефисного написания двусловных иноязычных имен собственных, переданных кириллицей. Район Вест(?)Энд, театр «Донмар(?)Уэрхаус», центр «Сэйнт(?)Эннс(?)Уэрхаус». Спасибо! P.S. Очень надеюсь, что в этот раз удастся до вас достучаться.

Ответ справочной службы русского языка

1. Ответ на Ваш вопрос о частеречной принадлежности дискуссионный. Однако мы считаем, что в приведенном контексте слово ограниченно по значению ближе к прилагательному. Ср.: количество ограниченно (т. е. мало) и время ограничено пятью минутами (т. е. его кто-то ограничил до 5 минут). В соответствии с правилами краткие прилагательные нужно писать с двумя н, если в соотносимой с ними полной форме пишется две н (ограниченный). Однако нельзя не признать, что практика письма столь сильно сопротивляется этому правилу (за исключением нескольких слов), что есть основания задуматься, не нуждается ли оно в корректировке. Подробный разбор этой проблемы выполнен в статье Е. В. Арутюновой и В. М. Пахомова «Правило востребован(н)о, но ограничен(н)о. К проблеме выбора одной и двух н в кратких формах прилагательных» в pdf»>сборнике научных статей по материалам Тотального диктанта.

2. Подобные написания устанавливаются в словарном порядке.

Здравствуйте! Подскажите, пожалуйста, склоняется ли аббревиатура ГУЛАГ в названии книги Солженицына? Приступил к работе над «Архипелагом ГУЛАГ» или Приступил к работе над «Архипелагом ГУЛАГом»?

Ответ справочной службы русского языка

Правильно: над «Архипелагом ГУЛАГом». См. pdf»>издание, подготовленное Н. Д. Солженицыной.

Здравствуйте, пытаясь разобраться в причинах дефисного написания прилагательных при подчиненном соотношении основ, я наткнулась на следующее ваше объяснение: «Из правила о дефисном/слитном написании прилагательных в зависимости от соотношения основ (сочинение/подчинение) в современной письменной речи имеется очень много исключений. Фактически это правило во многих случаях не работает. Дефисному написанию прилагательных способствует наличие в первой основе суффикса относительных прилагательных -н-, -енн-, -ов-, -ск-: естественно-научный, военно-исторический, электронно-лучевой, стрелково-спортивный, гражданско-правовой, парашютно-десантный и мн. др. Число таких «неправильных» написаний неуклонно растет в течение последних десятилетий. Во всех спорных случаях следует обращаться к орфографическим словарям». Не могли бы вы также дать ссылку на нормативные издания, фиксирующие эту тенденцию?

Ответ справочной службы русского языка

Об исследовании орфографии сложных прилагательных можно прочитать, например, в статье Букчиной Б. З. и Калакуцкой Л. П. «Лингвистические основания орфографического оформления сложных слов» (см.: Нерешенные вопросы русского правописания / ред. коллегия: Р. И. Аванесов, Л. П. Калакуцкая, А. А. Реформатский ; АН СССР, Ин-т русского языка. М. : «Наука», 1974. С. 5–14); в предисловии к словарю Букчиной Б. З. и Калакуцкой Л. П. «Слитно или раздельно?» (1-е изд. М., 1972; 3-е изд., испр. и доп. М. 1982), в книге Бешенковой Е. В. и Ивановой О. Е. «Правила русской орфографии с комментариями» (Тамбов, 2012. С. 86–87; см. pdf»>здесь).

Добрый день. Являюсь инженером. Часто приходится переводить чертежи и документы в формат pdf. Для этого коллегами используются слова: «запдфь», «спдфнуть», «спдфь» (ни одной гласной!), «перепдфь» в различных вариантах написания и произношения. Эти слова используются повсеместно людьми, которые хотя бы сталкивались раз с печатью документов, т.е. людьми различных профессий и социальных групп. Везде можно услышать подобные слова. Как согласно правилам русского языка использовать такие слова? Можно ли их отнести к профессиональному жаргону, если они не «привязаны» ни к какой профессии и даже отрасли? Существует ли элегантные и лаконичные выражения, описывающие процесс печати документов в формате pdf на основе предоставленных файлов?.

Ответ справочной службы русского языка

В явлении, которое Вы описываете, проявляется важный закон языка – закон экономии. В соответствии с литературной нормой нужно говорить: распечатать файл в формате pdf, перевести документ в формат pdf и т. п. Но эти выражения слишком длинны, и вот язык рождает новые, необходимые для быстрой коммуникации слова. Эти слова относятся к профессиональному жаргону и произносятся с гласными – по законам русской фонетики: [спэдээф’], [п’эр’эпэдээф’ит’].

Portable Document Format (PDF) is an open standard for document exchange. The file format created by Adobe Systems in 1993 is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system.^[2]
Each PDF file encapsulates a complete description of a fixed-layout 2D document that includes the text, fonts, images, and 2D vector graphics which compose the documents. Lately, 3D drawings can be embedded to PDF documents with Acrobat 3D using U3D or PRC and various other data formats.^[3][4]

In 1991 Adobe Systems co-founder John Warnock outlined a system called «Camelot»,^[5] that evolved into the Portable Document Format (PDF) file-format.

Formerly a proprietary format, PDF was officially released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO/IEC 32000-1:2008.^[6][1]

History

PDF’s adoption in the early days of the format’s history was slow.^[7] Adobe Acrobat, Adobe’s suite for reading and creating PDFs, was not freely available; early versions of PDF had no support for external hyperlinks, reducing its usefulness on the Internet; the additional size of the PDF document compared to plain text meant significantly longer download times over the slower modems common at the time, and rendering the files was slow on less powerful machines. Additionally, there were competing formats such as Envoy, Common Ground Digital Paper, Farallon Replica and even Adobe’s own PostScript format (.ps); in those early years, the PDF file was mainly popular in desktop publishing workflow. In 1995, AT&T Labs commenced work on another electronic document standard targeted at libraries and archives for preserving their books and documents, DjVu. This standard has evolved into the .djv/ .djvu format, which has had growing success and penetration in the online world for eBooks, catalogs, and image-sharing.

Adobe soon started distributing its Acrobat Reader (now Adobe Reader) program at no cost, and continued supporting the original PDF, which eventually became the de facto standard for printable documents on the web (a standard web document).

The PDF file format has changed several times and continues to evolve, as new versions of Adobe Acrobat were released. There have been nine versions of PDF with corresponding Acrobat releases:^[8]

• (1993) – PDF 1.0 / Acrobat 1.0
• (1994) – PDF 1.1 / Acrobat 2.0
• (1996) – PDF 1.2 / Acrobat 3.0
• (1999) – PDF 1.3 / Acrobat 4.0
• (2001) – PDF 1.4 / Acrobat 5.0
• (2003) – PDF 1.5 / Acrobat 6.0
• (2005) – PDF 1.6 / Acrobat 7.0
• (2006) – PDF 1.7 / Acrobat 8.0
• (2008) – PDF 1.7, Adobe Extension Level 3 / Acrobat 9.0
• (2009) – PDF 1.7, Adobe Extension Level 5 / Acrobat 9.1

The ISO 32000-1:2008 PDF open standard was published by the ISO on July 1, 2008.
PDF is now a published ISO standard, titled Document management—Portable document format—Part 1: PDF 1.7

According to the ISO PDF standard abstract:

ISO 32000-1:2008 specifies a digital form for representing electronic documents to enable users to exchange and view electronic documents independent of the environment in which they were created or the environment in which they are viewed or printed. It is intended for the developer of software that creates PDF files (conforming writers), software that reads existing PDF files and interprets their contents for display and interaction (conforming readers) and PDF products that read and/or write PDF files for a variety of other purposes (conforming products).

A new version named «PDF 2.0» is currently under development (ISO/NP 32000-2 — Document management—Portable document format—Part 2: PDF 2.0).^[9]

Technical foundations

Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.^[10]

The PDF combines three technologies:

• A subset of the PostScript page description programming language, for generating the layout and graphics.
• A font-embedding/replacement system to allow fonts to travel with the documents.
• A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.

PostScript

PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. PDF is a file format, not a programming language, so that flow control commands such as if and loop are removed, while graphics commands such as lineto remain.

Often, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized; any files, graphics, or fonts to which the document refers also are collected; then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.

As a document format, PDF has several advantages over PostScript:

• PDF contains tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
• PDF (from version 1.4) supports true graphic transparency; PostScript does not.
• PostScript is an imperative programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages in a PostScript document must be processed in order to determine the correct appearance of a given page, whereas each page in a PDF document is unaffected by the others. As a result, PDF viewers allow the user to quickly jump to the final pages of a long document, whereas a Postscript viewer needs to process all pages sequentially before being able to display the destination page (unless the optional PostScript Document Structuring Conventions have been carefully complied with).

Technical overview

File structure

A PDF file consists primarily of objects, of which there are eight types:^[11]

• Boolean values, representing true or false
• Numbers
• Strings
• Names
• Arrays, ordered collections of objects
• Dictionaries, collections of objects indexed by Names
• Streams, usually containing large amounts of data
• The null object

Objects may be either direct (embedded in another object) or indirect. Indirect objects are numbered with an object number and a generation number. An index table called the xref table gives the byte offset of each indirect object from the start of the file.^[12] This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (incremental update). Beginning with PDF version 1.5, indirect objects may also be located in special streams known as object streams. This technique reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF.

There are two layouts to the PDF files—non-linear (not «optimized») and linear («optimized»). Non-linear PDF files consume less disk space than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file. Linear PDF files (also called «optimized» or «web optimized» PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since they are written to disk in a linear (as in page order) fashion.^[13] PDF files may be optimized using Adobe Acrobat software or pdfopt, which is part of GPL Ghostscript.

Imaging model

The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.

PDF graphics use a device independent Cartesian coordinate system to describe the surface of a page. A PDF page description can use a matrix to scale, rotate, or skew graphical elements. A key concept in PDF is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description. PDF has (as of version 1.6) 24 graphics state properties, of which some of the most important are:

• The current transformation matrix (CTM), which determines the coordinate system
• The clipping path
• The color space
• The alpha constant, which is a key component of transparency

Vector graphics

Vector graphics in PDF, as in PostScript, are constructed with paths. Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text. Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves. Paths can be stroked, filled, or used for clipping. Strokes and fills can use any color set in the graphics state, including patterns.

PDF supports several types of patterns. The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly. This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn. Beginning with PDF 1.3 there is also a shading pattern, which draws continuously varying colors. There are seven types of shading pattern of which the simplest are the axial shade (Type 2) and radial shade (Type 3).

Raster images

Raster images in PDF (called Image XObjects) are represented by dictionaries with an associated stream. The dictionary describes properties of the image, and the stream contains the image data. (Less commonly, a raster image may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes. Image filters supported in PDF include the general purpose filters

• ASCII85Decode a deprecated filter used to put the stream into 7-bit ASCII
• ASCIIHexDecode similar to ASCII85Decode but less compact
• FlateDecode a commonly used filter based on the DEFLATE or Zip algorithm
• LZWDecode a deprecated filter based on LZW Compression
• RunLengthDecode a simple compression method for streams with repetitive data using the Run-length encoding algorithm

and the image-specific filters

• DCTDecode a lossy filter based on the JPEG standard
• CCITTFaxDecode a lossless filter based on the CCITT fax compression standard
• JBIG2Decode a lossy or lossless filter based on the JBIG2 standard, introduced in PDF 1.4
• JPXDecode a lossy or lossless filter based on the JPEG 2000 standard, introduced in PDF 1.5

Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of external streams or Alternate Images. Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these techniques.

Raster images can be exported using applications such as pdfimages from the xpdf/poppler package,^[14] or using the «Export All Images» function in the «Advanced» menu of Adobe Acrobat,^[15] version 6 or later.^[16]

Text

Text in PDF is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource.

Fonts

A font object in PDF is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded font file. The latter case is called an embedded font while the former is called an unembedded font. The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType. Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.

Encodings

Within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding. There are a number of predefined encodings, including WinAnsi, MacRoman, and a large number of encodings for East Asian languages, and a font can have its own built-in encoding. (Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font’s built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts).^[17] The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.

For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for horizontal writing) and Identity-V (for vertical) are used. With such fonts it is necessary to provide a ToUnicode table if semantic information about the characters is to be preserved.

Transparency

The original imaging model of PDF was, like PostScript’s, opaque: each object drawn on the page completely replaced anything previously marked in the same location. In PDF 1.4 the imaging model was extended to allow transparency. When transparency is used, new objects interact with previously marked objects to produce blending effects. The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to the PDF 1.3 and earlier specifications. As a result, files that use a small amount of transparency might view acceptably in older viewers, but files making extensive use of transparency could view completely wrongly in an older viewer without warning.

The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha. The model is closely aligned with the features of Adobe Illustrator version 9. The blend modes were based on those used by Adobe Photoshop at the time. When the PDF 1.4 specification was published the formulas for calculating blend modes were kept secret by Adobe. They have since been published.^[18]

The concept of a transparency group in PDF specification is independent of existing notions of «group» or «layer» in applications such as Adobe Illustrator. Those groupings reflect logical relationships among objects that are meaningful when editing those objects,
but they are not part of the imaging model.

Interactive elements

PDF files may contain interactive elements such as annotations and form fields.

Interactive Forms is a mechanism to add forms to the PDF file format.

PDF currently supports two different methods for integrating data and PDF forms. Both formats today coexist in PDF specification:^{[19][20][21][22][23]}

• AcroForms (also known as Acrobat forms), introduced in the PDF 1.2 format specification and included in all later PDF specifications.
• Adobe XML Forms Architecture (XFA) forms, introduced in the PDF 1.5 format specification. The XFA specification is not included in the PDF specification, it is only referenced as an optional feature. Adobe XFA Forms are not compatible with AcroForms.^[19][24]

AcroForms

AcroForms were introduced in the PDF 1.2 format.^[25] AcroForms permit using objects (text boxes, radiobuttons, etc.) and some code (JavaScript).

Alongside the standard PDF action types, Interactive forms (AcroForms) support submitting, resetting, and importing data. Submit action transmits the names and values of selected interactive form fields to a specified uniform resource locator (URL). Interactive form field names and values may be submitted in any of the following formats, (depending on the settings of the action’s ExportFormat, SubmitPDF, and XFDF flags):^[20]

• HTML Form format (HTML 4.01 Specification since PDF 1.5; HTML 2.0 since 1.2)
• Forms Data Format (FDF)
• XML Forms Data Format (XFDF) (external XML Forms Data Format Specification, Version 2.0; supported since PDF 1.5; it replaced the «XML» form submission format defined in PDF 1.4.)
• PDF (the entire document can be submitted rather than individual fields and values). (defined in PDF 1.4)

AcroForms can keep form field values in external stand-alone files containing key:value pairs. The external files may use Forms Data Format (FDF) and XML Forms Data Format (XFDF) files.^[26][27][28] The usage rights (UR) signatures define rights for import form data files in FDF, XFDF and text (CSV/TSV) formats, and export form data files in FDF and XFDF formats.^[20]

Forms Data Format (FDF)

The Forms Data Format (FDF) is based on PDF, it uses the same syntax and has essentially the same file structure, but is much simpler than PDF, since the body of an FDF document consists of only one required object. Forms Data Format is defined in PDF format specification (since PDF 1.2). The Forms Data Format can be used when submitting form data to a server, receiving the response, and incorporating into the interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. Beginning in PDF 1.3, FDF can be used to define a container for annotations that are separate from the PDF document to which they apply. FDF is typically used to encapsulate information such as X.509 certificates, requests for certificates, directory settings, timestamp server settings, and embedded PDF files for network transmission.^[28] The FDF uses the MIME content type application/vnd.fdf, filename extension .fdf and on Mac OS it uses file type ‘FDF ‘.^[20] A support for importing and exporting FDF stand-alone files is not widely implemented in free or freeware PDF software. For example, there is no support in Evince, Okular, KPDF or Sumatra PDF. Import support for stand-alone FDF files is implemented in Adobe Reader; export and import support (including saving of FDF data in PDF) is for example implemented in Foxit Reader and PDF-XChange Viewer Free; saving of FDF data in a PDF file is also supported in pdftk.

XML Forms Data Format (XFDF)

XML Forms Data Format (XFDF) is the XML version of Forms Data Format, but the XFDF implements only a subset of FDF containing forms and annotations. There are not XFDF equivalents for some entries in the FDF dictionary — such as the Status, Encoding, JavaScript, Pages keys, EmbeddedFDFs, Differences and Target. The XFDF specification is referenced (but not included) in PDF 1.5 specification (and in later versions). It is described separately in XML Forms Data Format Specification^[27]. The PDF 1.4 specification allowed form submissions in XML format, but this was replaced by submissions in XFDF format in PDF 1.5 specification. XFDF conforms to the XML standard. XFDF can be used the same way as FDF — e.g. form data is submitted to a server, modifications are made, then sent back and the new form data is imported in an interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. A support for importing and exporting FDF stand-alone files is not widely implemented in free or freeware PDF software. Import of XFDF is implemented in Adobe Reader 5 and later versions; import and export is implemented in PDF-XChange Viewer Free; embedding of XFDF data in PDF form is implemented in pdftk (pdf toolkit).

Adobe XML Forms Architecture (XFA)

Main article: XML Forms Architecture

In the PDF 1.5 format, Adobe Systems introduced a new, proprietary format for forms, namely Adobe XML Forms Architecture (XFA) forms. The XFA 2.02 is referenced in the PDF 1.5 specification (and also in later versions) but is described separately in Adobe XML Forms Architecture (XFA) Specification, which has several versions.^[25][31] Adobe XFA Forms are not compatible with AcroForms. Creating XFA Forms for use in Adobe Reader requires Adobe LiveCycle Forms Designer.^[19][32] Adobe Reader contains «disabled features» for use of XFA Forms, that will activate only when opening a PDF document that was created using enabling technology available only from Adobe.^[33][34] The XFA Forms are not compatible with Adobe Reader prior to version 6.^[19]

XFA forms can be created and used as PDF files or as XDP (XML Data Package) files. The format of an XFA resource in PDF is described by the XML Data Package Specification.^[20] The XDP may be a standalone document or it may in turn be carried inside a PDF document. XDP provides a mechanism for packaging form components within a surrounding XML container. An XDP can also package a PDF file, along with XML form and template data.^[31] PDF may contain XFA (in XDP format), but also XFA may contain PDF.^[31] When the XFA (XML Forms Architecture) grammars used for an XFA form are moved from one application to another, they must be packaged as an XML Data Package.^[35]

When the PDF and XFA are combined, the result is a form in which each page of the XFA form overlays a PDF background. This architecture is
sometimes referred to as XFAF (XFA Foreground). The alternative is to express all of the form, including boilerplate, directly in XFA. It is sometimes called full XFA.^[35]

Starting with PDF 1.5, the text contents of variable text form fields, as well as markup annotations may include formatting information (style information). These rich text strings are XML documents that conform to the rich text conventions specified for the XML Forms Architecture specification 2.02, which is itself a subset of the XHTML 1.0 specification, augmented with a restricted set of CSS2 style attributes.^[20]
In PDF 1.6, PDF supports the rich text elements and attributes specified in the XML Forms Architecture (XFA) Specification, 2.2.
In PDF 1.7, PDF supports the rich text elements and attributes specified in the XML Forms Architecture (XFA) Specification, 2.4^[20]

Logical structure and accessibility

A PDF may contain structure information to enable better text extraction and accessibility. When published, PDF/UA, now ISO/AWI 14289, will provide definitive information on how the contents of PDF files are to be tagged with accurate structure information.

Security and signatures

A PDF file may be encrypted for security, or digitally signed for authentication.

The standard security provided by Acrobat PDF consists of two different methods and two different passwords, «user password» and «owner password». A PDF document may be protected by password to open (‘user’ password) and the document may also specify operations that should be restricted even when the document is decrypted: printing; copying text and graphics out of the document; modifying the document; and adding or modifying text notes and AcroForm fields (using ‘owner’ password). However, all operations (except the document open password protection, if applicable) which are restricted by «owner» or «user» passwords are trivially circumvented by many commonly available «PDF cracking» software and even freely online,^[36] and thus these restrictions are obviously ineffective in letting the author control what can and cannot be done with the pdf file he or she created, once it is distributed. This warning is also displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.

Even without removing the password, most freeware or open source PDF readers will ignore the digital rights management «protections» and will allow the user to print or make copy of excerpts of the text as if the document were not limited with such anti-features.

Usage rights

Beginning with PDF 1.5, Usage rights (UR) signatures are used to enable additional interactive features that are not available by default in a particular PDF viewer application. The signature is used to validate that the permissions have been granted by a bonafide granting authority. For example, it can be used to allow a user:^[20]

• to save the PDF document along with modified form and/or annotation data
• import form data files in FDF, XFDF and text (CSV/TSV) formats
• export form data files in FDF and XFDF formats
• submit form data
• instantiate new pages from named page templates
• apply a digital signature to existing digital signature form field
• create, delete, modify, copy, import, export annotations

For example, Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. Adobe Reader will verify that the signature uses a certificate from an Adobe-authorized certificate authority. The PDF 1.5 specification declares that other PDF viewer applications are free to use this same mechanism for their own purposes.^[20]

File attachments

This section requires expansion. (August 2008)

PDF files can have document-level and page-level file attachments, which the reader can access and open or save to their local filesystem. PDF attachments can be added to existing PDF files for example using pdftk. Adobe Reader provides support for attachments, and poppler based readers like Evince or Okular also have some support for document-level attachments.

Metadata

PDF files can contain two types of metadata.^[37] The first is the Document Information Dictionary, a set of key/value fields such as author, title, subject, creation and update dates. This is stored in the optional Info trailer of the file. A small set of fields is defined, and can be extended with additional text values if required.

Later, in PDF 1.4, support was added for the Metadata Streams, using the Extensible Metadata Platform (XMP) to add XML standards-based extensible metadata as used in other file formats. This allows metadata to be attached to any stream in the document, such as information about embedded illustrations, as well as the whole document (attaching to the document catalog), using an extensible schema.

Subsets

Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:

• PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
• PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
• PDF/E for exchange of engineering drawings (work done in ISO TC171)
• PDF/UA for universally accessible PDF files

A PDF/H variant (PDF for Healthcare) is being developed.^[38] However, it may consist more of a set of «best practices» than of a specific format or subset.

Mars

See also: Page description markup language

Adobe is exploring an XML-based next-generation PDF code named Mars.^[39] Information about the Mars file format is published by Adobe at http://www.adobe.com/go/mars and also http://labs.adobe.com/wiki/index.php/Mars.

The format of graphic elements of Mars is sometimes described simply as «SVG«,^{[citation needed]} but according to the version 0.8 draft specification of November 2007 (§3 Mars SVG Support) the format is actually merely similar to SVG: it contains both additions to and subtractions from SVG, so it is in general neither viewable by nor creatable with standard SVG tools: some things will look noticeably different between SVG viewers and Mars viewers.

Technical issues

Accessibility

PDF files can be created specifically to be accessible for disabled people.^{[40][41][42][43][44]} Current PDF file formats can include tags (XML), text equivalents, captions, audio descriptions, et cetera. Some software can automatically produce tagged PDFs, however this feature is not always enabled by default. Leading screen readers, including JAWS, Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs aloud.^[45][46][47] Moreover, tagged PDFs can be re-flowed and magnified for readers with visual impairments. Problems remain with adding tags to older PDFs and those that are generated from scanned documents. In these cases, accessibility tags and re-flowing are unavailable, and must be created either manually or with OCR techniques. These processes are inaccessible to some disabled people. PDF/UA, the PDF/Universal Accessibility Committee, an activity of AIIM, is working on a specification for PDF accessibility based on ISO 32000.

One of the significant challenges with PDF accessibility is that PDF documents have three distinct views, which, depending on the document’s creation, can be inconsistent with each other. The three views are (i) the physical view, (ii) the tags view, and (iii) the content view. The physical view is displayed and printed (what most people consider a PDF document). The tags view is what screen readers read (useful for people with poor eyesight). The content view is displayed when the document is re-flowed to Acrobat (useful for people with mobility disability). For a PDF document to be accessible, the three views must be consistent with each other.^[48]

Security

PDF format attachments carrying viruses were first discovered in 2001. This virus, named «OUTLOOK.PDFWorm» or «Peachy», uses Microsoft Outlook to send itself as an attachment to an Adobe PDF file. It was activated with Adobe Acrobat, but not with Acrobat Reader.^[49]

From time to time, new vulnerabilities are discovered^[50] in various versions of Adobe Reader, prompting the company to issue security fixes. One aggravating factor is that Adobe Reader is by default integrated into browsers, and can be started automatically if the web page has an embedded PDF file, opening up a new vector of attack. If a malicious web page contains an infected PDF file that takes advantage of some vulnerability in Adobe Reader, the system is compromised even if the browser is up-to-date.

On March 30, 2010 security researcher Didier Stevens reported an «exploit» that causes an arbitrary executable to be run when a PDF file is opened, after the user accepts a warning prompt. The exploit works in several different PDF viewers including Adobe Reader and Foxit Reader.^[51]

Usage restrictions and monitoring

PDFs may be encrypted so that a password is needed to view or edit the contents. The PDF Reference defines both 40-bit and 128-bit encryption, both making use of a complex system of RC4 and MD5. The PDF Reference also defines ways in which third parties can define their own encryption systems for use in PDF.

PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing. The restrictions on copying, editing, or printing depend on the reader software to obey them, so the security they provide is limited. Printable documents especially might be saved instead as bitmaps and subject to OCR.

The PDF Reference has technical details or see ^[52] for an end-user overview. Like HTML files, PDF files may submit information to a web server. This could be used to track the IP address of the client PC, a process known as phoning home. After update 7.0.5 to Acrobat Reader, the user will be notified «via a dialogue box that the author of the file is auditing usage of the file, and be offered the option of continuing.»^[53]

Through its LiveCycle Policy Server product, Adobe provides a method to set security policies on specific documents. This can include requiring a user to authenticate and limiting the timeframe a document can be accessed or amount of time a document can be opened while offline. Once a PDF document is tied to a policy server and a specific policy, that policy can be changed or revoked by the owner. This controls documents that are otherwise «in the wild.» Each document open and close event can also be tracked by the policy server. Policy servers can be set up privately or Adobe offers a public service through Adobe Online Services.

Missing PostScript features

Compared to the PostScript format, PDF lacks e.g. the notion of «tray selection»; this can be used to indicate that some pages of a document must be printed on a different type of paper.

Such features are not omissions from the PDF format, whose scope only covers electronic documents. The JDF standard covers such aspects; however, it is a complex standard, which as of 2010 is not widely implemented. This hinders the replacement of PostScript by PDF.

Default display settings

PDF documents can contain display settings, including the page display layout and zoom level. Adobe Reader will use these settings to override the user’s default settings when opening the document.^[54] The free Adobe Reader cannot remove these settings.

Content

A PDF file is often a combination of vector graphics, text, and raster graphics. The basic types of content in a PDF are:

• text stored as such
• vector graphics for illustrations and designs that consist of shapes and lines
• raster graphics for photographs and other types of image

In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.

PDF 1.6 supports interactive 3D documents embedded in the PDF — 3D drawings can be embedded using U3D or PRC and various other data formats.^[3][4]

Two PDF files that look similar on a computer screen may be of very different sizes. For example, a high resolution raster image takes more space than a low resolution one. Typically higher resolution is needed for printing documents than for displaying them on screen. Other things that may increase the size of a file is embedding full fonts, especially for Asiatic scripts, and storing text as graphics.

Standard Type 1 Fonts

There are fourteen typefaces that have a special significance to PDF documents:

• Times (v3) (in regular, italic, bold, and bold italic)
• Courier (in regular, oblique, bold and bold oblique)
• Helvetica (v3) (in regular, oblique, bold and bold oblique)
• Symbol
• Zapf Dingbats

These fonts, sometimes referred to as the «base fourteen fonts»^[55] should always be present (actually present or a close substitute) and so need not be embedded in a PDF.^[56] PDF viewers must know about the metrics of these fonts. Other fonts may be substituted if they are not embedded in a PDF.

Versions

Version	Year of publication	New features	Supported version
1.0	1993		Acrobat Reader (Carousel)
1.1	1996	Passwords, encryption (MD5, RC4 40bit), device-independent color, threads and links[57]	Acrobat Reader 2.0
1.2	1996	Interactive page elements (such as radio buttons and checkboxes); interactive, fill-in forms (AcroForm); Forms Data Format (FDF) for interactive form data that can be imported, exported, transmitted and received from the Web; mouse events; support for playing movies from external files; support for playing sounds, either embedded in the PDF file or from external files ; Unicode; advanced color features and image proxying[57]	Acrobat Reader 3.0
1.3	2000	Digital signatures; ICC and DeviceN color spaces; JavaScript actions; Embedded file streams — embedding files of any type directly within the body of the PDF file itself (e.g. used for attachments); many new annotation types; new features of the Adobe imaging model embodied in PostScript LanguageLevel 3; Masked images; Alternate representations for a single image; Smooth shading; Enhanced page numbering; Web Capture — a facility for capturing information from World Wide Web and converting it to PDF; Logical structure — a facility for representating logical structure independently of its graphical structure; additional support for CIDFonts; data structures for mapping strings and numbers to PDF objects; Prepress Support — information useful in prepress production workflows; new functions — for several types of function object that represent parameterized classes of functions[58][59]	Acrobat Reader 4.0
1.4	2001	JBIG2; transparency; RC4 encryption key lengths greater than 40bits (40-128bits); enhancements to interactive forms and Forms Data Format (FDF), support for XML form submissions, embedded FDF files, Unicode specification of field export values, support for remote collaboration and digital signatures in FDF files; support for accessibility to disabled users; metadata streams using XML — Extensible Metadata Platform (XMP); Tagged PDF ;Facilities for including printer’s marks; Support for the display and preview of production-related page boundaries; New predefined CMaps; Alternate Presentations — alternate ways in which the document may be viewed; The ability to import content from one PDF document into another; EmbeddedFiles entry in the PDF document’s name dictionary — a standard location for the embedded data[59][60]; OCR text layer[citation needed] || Acrobat Reader 5.0
1.5	2003	JPEG 2000; enhanced support for embedding and playback of multimedia; object streams; cross reference streams; XML Forms Data Format (XFDF) for interactive form submission (replaced the XML format in PDF 1.4); support for forms, rich text elements and attributes based on Adobe’s XML Forms Architecture (XFA) 2.02; public-key security handlers using PKCS#7 (introduced in PDF 1.3 but not documented in the PDF Reference until PDF 1.5), public-key encryption, permissions — usage rights (UR) signatures (does not require document encryption), PKCS#7 with SHA-1, RSA up to 4096-bits; security handler can use its own encryption and decryption algorithms; Optional Content — sections of content in a PDF document that can be selectively viewed or hidden by document authors or consumers — for items such as CAD drawings, layered artwork, maps, and multi-language documents; Alternate Presentations — the only type is slideshow — invoked by means of JavaScript actions (Adobe Reader supports only SVG 1.0)[59][61][62] Support for Windows 98 dropped. To view and print newer version PDFs, such as those at the IRS website, with older versions of Reader requires downloading in Google Docs «Quick View» simplified PDF format.	Adobe Reader 6.0
1.6	2004	3D artwork, e.g. support for Universal 3D file format; OpenType font embedding; support for XFA 2.2 rich text elements and attributes; AES encryption; PKCS#7 with SHA256, DSA up to 4096-bits; NChannel color spaces; additional support for embedded file attachments, including cross-document linking to and from embedded files; enhancements and clarifications to digital signatures related to usage rights and modification detection and prevention signatures[59]	Adobe Reader 7.0
1.7	2006	increased presentation of 3D artwork; support for XFA 2.4 rich text elements and attributes; presentation of multiple file attachments (portable collections); document requirements for a PDF consumer application; new string types — PDFDocEncoded string, ASCII string, byte string; PKCS#7 with SHA384, SHA512 and RIPEMD160	Adobe Reader 8
1.7 Extension Level 3	2008	256-bit AES encryption; incorporation of XFA Datasets into a file conforming PDF/A-2; RichMedia annotations — attaching Flash applications, video (including Flash video with H.264), audio, and other multimedia with expanded functionality, two-way scripting bridge between Flash and a conforming application; support for the rich text conventions described in XFA 2.5 and 2.6	Adobe Reader 9
1.7 Extension Level 5	2009	XFA 3.0	Adobe Reader 9.1

Implementations

PDF-viewing software is generally provided free of charge, including versions by Adobe Reader, Foxit Reader, PDF-XChange Viewer, Sorax Reader and others.

There are many software options for creating PDFs, including the PDF printing capabilities built in to Mac OS X and some versions of Linux, the multi-platform OpenOffice.org, Microsoft Office 2007 (if updated to SP2),^[63] WordPerfect since version 9, Free PDF XP ^[64] and numerous PDF print drivers for Microsoft Windows, the pdfTeX typesetting system, the DocBook PDF tools, applications developed around Ghostscript and Adobe Acrobat itself. Google’s online office suite Google Docs also allows for uploading, and saving to the PDF format.

Editing PDFs (structure)

There is also specialized software for editing PDF files, though the choices are much more limited and often expensive. As of version 0.46, Inkscape also allows PDF editing through an intermediate translation step involving Poppler.

Annotating PDFs

See also: Comparison of notetaking software and Category:Web annotation

Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, add notes to already created PDF files. One UNIX application available as free software (under the GNU General Public License) is PDFedit. Another GPL-licensed application native to the Linux environment is Xournal. Xournal allows for annotating in different fonts and colours, as well as a rule for quickly underlining and highlighting lines of text or paragraphs. Xournal also has a shape recognition tool for squares, rectangles and circles. In Xournal annotations may be moved, copied and pasted. The freeware Foxit Reader allows annotating but adds a watermark on each annotated page. The commercial version of the package does not have this limitation. Tracker Software’s PDF-XChange Viewer allows annotations and markups without restrictions in its freeware alternative. Apple’s Mac OS X’s integrated PDF viewer, Preview, does also enable annotations. For mobile annotation, iAnnotate PDF for the iPad and Aji Annotate for the iPhone, both produced by Aji, allow annotation of PDFs as well as exporting summaries of the annotations.

There are also web annotation systems which allow to annotate pdf and other documents formats, e.g. A.nnotate, crocodoc, WebNotes.

Other applications and functionalities

Several applications embracing the PDF standard are now available as an online service including Scribd for viewing and storing, Pdfvue for online editing, and Zamzar for PDF Conversion.

In 1993 the Jaws RIP from Global Graphics became the first shipping prepress RIP that interpreted PDF natively without conversion to another format. The company released an upgrade to their Harlequin RIP with the same capability in 1997.^{[citation needed]}

Agfa-Gevaert introduced and shipped Apogee, the first prepress workflow system based on PDF, in 1997.

Many commercial offset printers have accepted the submission of press-ready PDF files as a print source, specifically the PDF/X-1a subset and variations of the same.^[65] The submission of press-ready PDF files are a replacement for the problematic need for receiving collected native working files.

PDF was selected as the «native» metafile format for Mac OS X, replacing the PICT format of the earlier Mac OS. The imaging model of the Quartz graphics layer is based on the model common to Display PostScript and PDF, leading to the nickname «Display PDF». The Preview application can display PDF files, as can version 2.0 and later of the Safari web browser. System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the Print command. The files are then exported in PDF 1.3 format according to the file header. When taking a screenshot under Mac OS X versions 10.0 through 10.3, the image was also captured as a PDF; in 10.4 and 10.5 the default behaviour is set to capture as a PNG file, though this behaviour can be set back to PDF if required.

Some desktop printers also support direct PDF printing, which can interpret PDF data without external help. Currently, all PDF capable printers also support PostScript, but most PostScript printers do not support direct PDF printing.

The Free Software Foundation considers one of their high priority projects to be «developing a free, high-quality and fully functional set of libraries and programs that implement the PDF file format and associated technologies to the ISO 32000 standard.»^[66][67] The GNUpdf library has, however, not been released yet, while Poppler has enjoyed wider use in applications such as Evince, which comes with the GNOME desktop environment, at the expense of relying on the GPLv2-licensed Xpdf^[68][69] code base that can’t be used by GPLv3 programs.

The Apache PDFBox project of the Apache Software Foundation is an open source Java library for working with PDF documents. PDFBox is licensed under the Apache License.^[70]

References

External links

• Edit or make changes to a PDF file at Microsoft Support
  • Add a PDF to your Office file
  • Save PowerPoint presentations as PDF files
  • Save As or convert a publication to .pdf or .xps using Publisher
  • Word Help & Training: Edit a PDF
  • PDF reader in Microsoft Edge at Microsoft Learn
  • Adobe Acrobat for Microsoft 365
• Adobe: PostScript vs. PDF – Official introductory comparison of PS, EPS vs. PDF^{[dead link]}
  • Adobe PDF 101: Quick overview of PDF^{[dead link]}
  • Latest PDF version specification (including free version of the ISO standard)
  • PDF Reference Books: PDF 1.6 (ISBN 0-321-30474-8), PDF 1.4 (ISBN 0-201-75839-3), PDF 1.3 (ISBN 0-201-61588-6)
• ISO 19005-1:2005 Document of the PDF/A-1 Standard at the International Organization for Standardization (chargeable)
• Portable Document Format: An Introduction for Programmers – Quick one-page intro to PDF vs. PostScript and PDF internals (up to v1.3)
• Free PDF files hosting service
• Portable Document Format at Wikipedia

Portable Document Format (PDF) is an open standard for document exchange. The file format created by Adobe Systems in 1993 is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system.^[2]
Each PDF file encapsulates a complete description of a fixed-layout 2D document that includes the text, fonts, images, and 2D vector graphics which compose the documents. Lately, 3D drawings can be embedded to PDF documents with Acrobat 3D using U3D or PRC and various other data formats.^[3][4]

In 1991 Adobe Systems co-founder John Warnock outlined a system called «Camelot»,^[5] that evolved into the Portable Document Format (PDF) file-format.

Formerly a proprietary format, PDF was officially released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO/IEC 32000-1:2008.^[6][1]

History

PDF’s adoption in the early days of the format’s history was slow.^[7] Adobe Acrobat, Adobe’s suite for reading and creating PDFs, was not freely available; early versions of PDF had no support for external hyperlinks, reducing its usefulness on the Internet; the additional size of the PDF document compared to plain text meant significantly longer download times over the slower modems common at the time, and rendering the files was slow on less powerful machines. Additionally, there were competing formats such as Envoy, Common Ground Digital Paper, Farallon Replica and even Adobe’s own PostScript format (.ps); in those early years, the PDF file was mainly popular in desktop publishing workflow. In 1995, AT&T Labs commenced work on another electronic document standard targeted at libraries and archives for preserving their books and documents, DjVu. This standard has evolved into the .djv/ .djvu format, which has had growing success and penetration in the online world for eBooks, catalogs, and image-sharing.

Adobe soon started distributing its Acrobat Reader (now Adobe Reader) program at no cost, and continued supporting the original PDF, which eventually became the de facto standard for printable documents on the web (a standard web document).

The PDF file format has changed several times and continues to evolve, as new versions of Adobe Acrobat were released. There have been nine versions of PDF with corresponding Acrobat releases:^[8]

• (1993) – PDF 1.0 / Acrobat 1.0
• (1994) – PDF 1.1 / Acrobat 2.0
• (1996) – PDF 1.2 / Acrobat 3.0
• (1999) – PDF 1.3 / Acrobat 4.0
• (2001) – PDF 1.4 / Acrobat 5.0
• (2003) – PDF 1.5 / Acrobat 6.0
• (2005) – PDF 1.6 / Acrobat 7.0
• (2006) – PDF 1.7 / Acrobat 8.0
• (2008) – PDF 1.7, Adobe Extension Level 3 / Acrobat 9.0
• (2009) – PDF 1.7, Adobe Extension Level 5 / Acrobat 9.1

The ISO 32000-1:2008 PDF open standard was published by the ISO on July 1, 2008.
PDF is now a published ISO standard, titled Document management—Portable document format—Part 1: PDF 1.7

According to the ISO PDF standard abstract:

ISO 32000-1:2008 specifies a digital form for representing electronic documents to enable users to exchange and view electronic documents independent of the environment in which they were created or the environment in which they are viewed or printed. It is intended for the developer of software that creates PDF files (conforming writers), software that reads existing PDF files and interprets their contents for display and interaction (conforming readers) and PDF products that read and/or write PDF files for a variety of other purposes (conforming products).

A new version named «PDF 2.0» is currently under development (ISO/NP 32000-2 — Document management—Portable document format—Part 2: PDF 2.0).^[9]

Technical foundations

Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.^[10]

The PDF combines three technologies:

• A subset of the PostScript page description programming language, for generating the layout and graphics.
• A font-embedding/replacement system to allow fonts to travel with the documents.
• A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.

PostScript

PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. PDF is a file format, not a programming language, so that flow control commands such as if and loop are removed, while graphics commands such as lineto remain.

Often, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized; any files, graphics, or fonts to which the document refers also are collected; then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.

As a document format, PDF has several advantages over PostScript:

• PDF contains tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
• PDF (from version 1.4) supports true graphic transparency; PostScript does not.
• PostScript is an imperative programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages in a PostScript document must be processed in order to determine the correct appearance of a given page, whereas each page in a PDF document is unaffected by the others. As a result, PDF viewers allow the user to quickly jump to the final pages of a long document, whereas a Postscript viewer needs to process all pages sequentially before being able to display the destination page (unless the optional PostScript Document Structuring Conventions have been carefully complied with).

Technical overview

File structure

A PDF file consists primarily of objects, of which there are eight types:^[11]

• Boolean values, representing true or false
• Numbers
• Strings
• Names
• Arrays, ordered collections of objects
• Dictionaries, collections of objects indexed by Names
• Streams, usually containing large amounts of data
• The null object

Objects may be either direct (embedded in another object) or indirect. Indirect objects are numbered with an object number and a generation number. An index table called the xref table gives the byte offset of each indirect object from the start of the file.^[12] This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (incremental update). Beginning with PDF version 1.5, indirect objects may also be located in special streams known as object streams. This technique reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF.

There are two layouts to the PDF files—non-linear (not «optimized») and linear («optimized»). Non-linear PDF files consume less disk space than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file. Linear PDF files (also called «optimized» or «web optimized» PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since they are written to disk in a linear (as in page order) fashion.^[13] PDF files may be optimized using Adobe Acrobat software or pdfopt, which is part of GPL Ghostscript.

Imaging model

The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.

PDF graphics use a device independent Cartesian coordinate system to describe the surface of a page. A PDF page description can use a matrix to scale, rotate, or skew graphical elements. A key concept in PDF is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description. PDF has (as of version 1.6) 24 graphics state properties, of which some of the most important are:

• The current transformation matrix (CTM), which determines the coordinate system
• The clipping path
• The color space
• The alpha constant, which is a key component of transparency

Vector graphics

Vector graphics in PDF, as in PostScript, are constructed with paths. Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text. Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves. Paths can be stroked, filled, or used for clipping. Strokes and fills can use any color set in the graphics state, including patterns.

PDF supports several types of patterns. The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly. This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn. Beginning with PDF 1.3 there is also a shading pattern, which draws continuously varying colors. There are seven types of shading pattern of which the simplest are the axial shade (Type 2) and radial shade (Type 3).

Raster images

Raster images in PDF (called Image XObjects) are represented by dictionaries with an associated stream. The dictionary describes properties of the image, and the stream contains the image data. (Less commonly, a raster image may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes. Image filters supported in PDF include the general purpose filters

• ASCII85Decode a deprecated filter used to put the stream into 7-bit ASCII
• ASCIIHexDecode similar to ASCII85Decode but less compact
• FlateDecode a commonly used filter based on the DEFLATE or Zip algorithm
• LZWDecode a deprecated filter based on LZW Compression
• RunLengthDecode a simple compression method for streams with repetitive data using the Run-length encoding algorithm

and the image-specific filters

• DCTDecode a lossy filter based on the JPEG standard
• CCITTFaxDecode a lossless filter based on the CCITT fax compression standard
• JBIG2Decode a lossy or lossless filter based on the JBIG2 standard, introduced in PDF 1.4
• JPXDecode a lossy or lossless filter based on the JPEG 2000 standard, introduced in PDF 1.5

Normally all image content in a PDF is embedded in the file. But PDF allows image data to be stored in external files by the use of external streams or Alternate Images. Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these techniques.

Raster images can be exported using applications such as pdfimages from the xpdf/poppler package,^[14] or using the «Export All Images» function in the «Advanced» menu of Adobe Acrobat,^[15] version 6 or later.^[16]

Text

Text in PDF is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource.

Fonts

A font object in PDF is a description of a digital typeface. It may either describe the characteristics of a typeface, or it may include an embedded font file. The latter case is called an embedded font while the former is called an unembedded font. The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType. Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.

Encodings

Within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding. There are a number of predefined encodings, including WinAnsi, MacRoman, and a large number of encodings for East Asian languages, and a font can have its own built-in encoding. (Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font’s built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts).^[17] The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.

For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for horizontal writing) and Identity-V (for vertical) are used. With such fonts it is necessary to provide a ToUnicode table if semantic information about the characters is to be preserved.

Transparency

The original imaging model of PDF was, like PostScript’s, opaque: each object drawn on the page completely replaced anything previously marked in the same location. In PDF 1.4 the imaging model was extended to allow transparency. When transparency is used, new objects interact with previously marked objects to produce blending effects. The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to the PDF 1.3 and earlier specifications. As a result, files that use a small amount of transparency might view acceptably in older viewers, but files making extensive use of transparency could view completely wrongly in an older viewer without warning.

The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha. The model is closely aligned with the features of Adobe Illustrator version 9. The blend modes were based on those used by Adobe Photoshop at the time. When the PDF 1.4 specification was published the formulas for calculating blend modes were kept secret by Adobe. They have since been published.^[18]

The concept of a transparency group in PDF specification is independent of existing notions of «group» or «layer» in applications such as Adobe Illustrator. Those groupings reflect logical relationships among objects that are meaningful when editing those objects,
but they are not part of the imaging model.

Interactive elements

PDF files may contain interactive elements such as annotations and form fields.

Interactive Forms is a mechanism to add forms to the PDF file format.

PDF currently supports two different methods for integrating data and PDF forms. Both formats today coexist in PDF specification:^{[19][20][21][22][23]}

• AcroForms (also known as Acrobat forms), introduced in the PDF 1.2 format specification and included in all later PDF specifications.
• Adobe XML Forms Architecture (XFA) forms, introduced in the PDF 1.5 format specification. The XFA specification is not included in the PDF specification, it is only referenced as an optional feature. Adobe XFA Forms are not compatible with AcroForms.^[19][24]

AcroForms

AcroForms were introduced in the PDF 1.2 format.^[25] AcroForms permit using objects (text boxes, radiobuttons, etc.) and some code (JavaScript).

Alongside the standard PDF action types, Interactive forms (AcroForms) support submitting, resetting, and importing data. Submit action transmits the names and values of selected interactive form fields to a specified uniform resource locator (URL). Interactive form field names and values may be submitted in any of the following formats, (depending on the settings of the action’s ExportFormat, SubmitPDF, and XFDF flags):^[20]

• HTML Form format (HTML 4.01 Specification since PDF 1.5; HTML 2.0 since 1.2)
• Forms Data Format (FDF)
• XML Forms Data Format (XFDF) (external XML Forms Data Format Specification, Version 2.0; supported since PDF 1.5; it replaced the «XML» form submission format defined in PDF 1.4.)
• PDF (the entire document can be submitted rather than individual fields and values). (defined in PDF 1.4)

AcroForms can keep form field values in external stand-alone files containing key:value pairs. The external files may use Forms Data Format (FDF) and XML Forms Data Format (XFDF) files.^[26][27][28] The usage rights (UR) signatures define rights for import form data files in FDF, XFDF and text (CSV/TSV) formats, and export form data files in FDF and XFDF formats.^[20]

Forms Data Format (FDF)

The Forms Data Format (FDF) is based on PDF, it uses the same syntax and has essentially the same file structure, but is much simpler than PDF, since the body of an FDF document consists of only one required object. Forms Data Format is defined in PDF format specification (since PDF 1.2). The Forms Data Format can be used when submitting form data to a server, receiving the response, and incorporating into the interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. Beginning in PDF 1.3, FDF can be used to define a container for annotations that are separate from the PDF document to which they apply. FDF is typically used to encapsulate information such as X.509 certificates, requests for certificates, directory settings, timestamp server settings, and embedded PDF files for network transmission.^[28] The FDF uses the MIME content type application/vnd.fdf, filename extension .fdf and on Mac OS it uses file type ‘FDF ‘.^[20] A support for importing and exporting FDF stand-alone files is not widely implemented in free or freeware PDF software. For example, there is no support in Evince, Okular, KPDF or Sumatra PDF. Import support for stand-alone FDF files is implemented in Adobe Reader; export and import support (including saving of FDF data in PDF) is for example implemented in Foxit Reader and PDF-XChange Viewer Free; saving of FDF data in a PDF file is also supported in pdftk.

XML Forms Data Format (XFDF)

XML Forms Data Format (XFDF) is the XML version of Forms Data Format, but the XFDF implements only a subset of FDF containing forms and annotations. There are not XFDF equivalents for some entries in the FDF dictionary — such as the Status, Encoding, JavaScript, Pages keys, EmbeddedFDFs, Differences and Target. The XFDF specification is referenced (but not included) in PDF 1.5 specification (and in later versions). It is described separately in XML Forms Data Format Specification^[27]. The PDF 1.4 specification allowed form submissions in XML format, but this was replaced by submissions in XFDF format in PDF 1.5 specification. XFDF conforms to the XML standard. XFDF can be used the same way as FDF — e.g. form data is submitted to a server, modifications are made, then sent back and the new form data is imported in an interactive form. It can also be used to export form data to stand-alone files that can be imported back into the corresponding PDF interactive form. A support for importing and exporting FDF stand-alone files is not widely implemented in free or freeware PDF software. Import of XFDF is implemented in Adobe Reader 5 and later versions; import and export is implemented in PDF-XChange Viewer Free; embedding of XFDF data in PDF form is implemented in pdftk (pdf toolkit).

Adobe XML Forms Architecture (XFA)

Main article: XML Forms Architecture

In the PDF 1.5 format, Adobe Systems introduced a new, proprietary format for forms, namely Adobe XML Forms Architecture (XFA) forms. The XFA 2.02 is referenced in the PDF 1.5 specification (and also in later versions) but is described separately in Adobe XML Forms Architecture (XFA) Specification, which has several versions.^[25][31] Adobe XFA Forms are not compatible with AcroForms. Creating XFA Forms for use in Adobe Reader requires Adobe LiveCycle Forms Designer.^[19][32] Adobe Reader contains «disabled features» for use of XFA Forms, that will activate only when opening a PDF document that was created using enabling technology available only from Adobe.^[33][34] The XFA Forms are not compatible with Adobe Reader prior to version 6.^[19]

XFA forms can be created and used as PDF files or as XDP (XML Data Package) files. The format of an XFA resource in PDF is described by the XML Data Package Specification.^[20] The XDP may be a standalone document or it may in turn be carried inside a PDF document. XDP provides a mechanism for packaging form components within a surrounding XML container. An XDP can also package a PDF file, along with XML form and template data.^[31] PDF may contain XFA (in XDP format), but also XFA may contain PDF.^[31] When the XFA (XML Forms Architecture) grammars used for an XFA form are moved from one application to another, they must be packaged as an XML Data Package.^[35]

When the PDF and XFA are combined, the result is a form in which each page of the XFA form overlays a PDF background. This architecture is
sometimes referred to as XFAF (XFA Foreground). The alternative is to express all of the form, including boilerplate, directly in XFA. It is sometimes called full XFA.^[35]

Starting with PDF 1.5, the text contents of variable text form fields, as well as markup annotations may include formatting information (style information). These rich text strings are XML documents that conform to the rich text conventions specified for the XML Forms Architecture specification 2.02, which is itself a subset of the XHTML 1.0 specification, augmented with a restricted set of CSS2 style attributes.^[20]
In PDF 1.6, PDF supports the rich text elements and attributes specified in the XML Forms Architecture (XFA) Specification, 2.2.
In PDF 1.7, PDF supports the rich text elements and attributes specified in the XML Forms Architecture (XFA) Specification, 2.4^[20]

Logical structure and accessibility

A PDF may contain structure information to enable better text extraction and accessibility. When published, PDF/UA, now ISO/AWI 14289, will provide definitive information on how the contents of PDF files are to be tagged with accurate structure information.

Security and signatures

A PDF file may be encrypted for security, or digitally signed for authentication.

The standard security provided by Acrobat PDF consists of two different methods and two different passwords, «user password» and «owner password». A PDF document may be protected by password to open (‘user’ password) and the document may also specify operations that should be restricted even when the document is decrypted: printing; copying text and graphics out of the document; modifying the document; and adding or modifying text notes and AcroForm fields (using ‘owner’ password). However, all operations (except the document open password protection, if applicable) which are restricted by «owner» or «user» passwords are trivially circumvented by many commonly available «PDF cracking» software and even freely online,^[36] and thus these restrictions are obviously ineffective in letting the author control what can and cannot be done with the pdf file he or she created, once it is distributed. This warning is also displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.

Even without removing the password, most freeware or open source PDF readers will ignore the digital rights management «protections» and will allow the user to print or make copy of excerpts of the text as if the document were not limited with such anti-features.

Usage rights

Beginning with PDF 1.5, Usage rights (UR) signatures are used to enable additional interactive features that are not available by default in a particular PDF viewer application. The signature is used to validate that the permissions have been granted by a bonafide granting authority. For example, it can be used to allow a user:^[20]

• to save the PDF document along with modified form and/or annotation data
• import form data files in FDF, XFDF and text (CSV/TSV) formats
• export form data files in FDF and XFDF formats
• submit form data
• instantiate new pages from named page templates
• apply a digital signature to existing digital signature form field
• create, delete, modify, copy, import, export annotations

For example, Adobe Systems grants permissions to enable additional features in Adobe Reader, using public-key cryptography. Adobe Reader will verify that the signature uses a certificate from an Adobe-authorized certificate authority. The PDF 1.5 specification declares that other PDF viewer applications are free to use this same mechanism for their own purposes.^[20]

File attachments

This section requires expansion. (August 2008)

PDF files can have document-level and page-level file attachments, which the reader can access and open or save to their local filesystem. PDF attachments can be added to existing PDF files for example using pdftk. Adobe Reader provides support for attachments, and poppler based readers like Evince or Okular also have some support for document-level attachments.

Metadata

PDF files can contain two types of metadata.^[37] The first is the Document Information Dictionary, a set of key/value fields such as author, title, subject, creation and update dates. This is stored in the optional Info trailer of the file. A small set of fields is defined, and can be extended with additional text values if required.

Later, in PDF 1.4, support was added for the Metadata Streams, using the Extensible Metadata Platform (XMP) to add XML standards-based extensible metadata as used in other file formats. This allows metadata to be attached to any stream in the document, such as information about embedded illustrations, as well as the whole document (attaching to the document catalog), using an extensible schema.

Subsets

Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:

• PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
• PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
• PDF/E for exchange of engineering drawings (work done in ISO TC171)
• PDF/UA for universally accessible PDF files

A PDF/H variant (PDF for Healthcare) is being developed.^[38] However, it may consist more of a set of «best practices» than of a specific format or subset.

Mars

See also: Page description markup language

Adobe is exploring an XML-based next-generation PDF code named Mars.^[39] Information about the Mars file format is published by Adobe at http://www.adobe.com/go/mars and also http://labs.adobe.com/wiki/index.php/Mars.

The format of graphic elements of Mars is sometimes described simply as «SVG«,^{[citation needed]} but according to the version 0.8 draft specification of November 2007 (§3 Mars SVG Support) the format is actually merely similar to SVG: it contains both additions to and subtractions from SVG, so it is in general neither viewable by nor creatable with standard SVG tools: some things will look noticeably different between SVG viewers and Mars viewers.

Technical issues

Accessibility

PDF files can be created specifically to be accessible for disabled people.^{[40][41][42][43][44]} Current PDF file formats can include tags (XML), text equivalents, captions, audio descriptions, et cetera. Some software can automatically produce tagged PDFs, however this feature is not always enabled by default. Leading screen readers, including JAWS, Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDFs; current versions of the Acrobat and Acrobat Reader programs can also read PDFs aloud.^[45][46][47] Moreover, tagged PDFs can be re-flowed and magnified for readers with visual impairments. Problems remain with adding tags to older PDFs and those that are generated from scanned documents. In these cases, accessibility tags and re-flowing are unavailable, and must be created either manually or with OCR techniques. These processes are inaccessible to some disabled people. PDF/UA, the PDF/Universal Accessibility Committee, an activity of AIIM, is working on a specification for PDF accessibility based on ISO 32000.

One of the significant challenges with PDF accessibility is that PDF documents have three distinct views, which, depending on the document’s creation, can be inconsistent with each other. The three views are (i) the physical view, (ii) the tags view, and (iii) the content view. The physical view is displayed and printed (what most people consider a PDF document). The tags view is what screen readers read (useful for people with poor eyesight). The content view is displayed when the document is re-flowed to Acrobat (useful for people with mobility disability). For a PDF document to be accessible, the three views must be consistent with each other.^[48]

Security

PDF format attachments carrying viruses were first discovered in 2001. This virus, named «OUTLOOK.PDFWorm» or «Peachy», uses Microsoft Outlook to send itself as an attachment to an Adobe PDF file. It was activated with Adobe Acrobat, but not with Acrobat Reader.^[49]

From time to time, new vulnerabilities are discovered^[50] in various versions of Adobe Reader, prompting the company to issue security fixes. One aggravating factor is that Adobe Reader is by default integrated into browsers, and can be started automatically if the web page has an embedded PDF file, opening up a new vector of attack. If a malicious web page contains an infected PDF file that takes advantage of some vulnerability in Adobe Reader, the system is compromised even if the browser is up-to-date.

On March 30, 2010 security researcher Didier Stevens reported an «exploit» that causes an arbitrary executable to be run when a PDF file is opened, after the user accepts a warning prompt. The exploit works in several different PDF viewers including Adobe Reader and Foxit Reader.^[51]

Usage restrictions and monitoring

PDFs may be encrypted so that a password is needed to view or edit the contents. The PDF Reference defines both 40-bit and 128-bit encryption, both making use of a complex system of RC4 and MD5. The PDF Reference also defines ways in which third parties can define their own encryption systems for use in PDF.

PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing. The restrictions on copying, editing, or printing depend on the reader software to obey them, so the security they provide is limited. Printable documents especially might be saved instead as bitmaps and subject to OCR.

The PDF Reference has technical details or see ^[52] for an end-user overview. Like HTML files, PDF files may submit information to a web server. This could be used to track the IP address of the client PC, a process known as phoning home. After update 7.0.5 to Acrobat Reader, the user will be notified «via a dialogue box that the author of the file is auditing usage of the file, and be offered the option of continuing.»^[53]

Through its LiveCycle Policy Server product, Adobe provides a method to set security policies on specific documents. This can include requiring a user to authenticate and limiting the timeframe a document can be accessed or amount of time a document can be opened while offline. Once a PDF document is tied to a policy server and a specific policy, that policy can be changed or revoked by the owner. This controls documents that are otherwise «in the wild.» Each document open and close event can also be tracked by the policy server. Policy servers can be set up privately or Adobe offers a public service through Adobe Online Services.

Missing PostScript features

Compared to the PostScript format, PDF lacks e.g. the notion of «tray selection»; this can be used to indicate that some pages of a document must be printed on a different type of paper.

Such features are not omissions from the PDF format, whose scope only covers electronic documents. The JDF standard covers such aspects; however, it is a complex standard, which as of 2010 is not widely implemented. This hinders the replacement of PostScript by PDF.

Default display settings

PDF documents can contain display settings, including the page display layout and zoom level. Adobe Reader will use these settings to override the user’s default settings when opening the document.^[54] The free Adobe Reader cannot remove these settings.

Content

A PDF file is often a combination of vector graphics, text, and raster graphics. The basic types of content in a PDF are:

• text stored as such
• vector graphics for illustrations and designs that consist of shapes and lines
• raster graphics for photographs and other types of image

In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.

PDF 1.6 supports interactive 3D documents embedded in the PDF — 3D drawings can be embedded using U3D or PRC and various other data formats.^[3][4]

Two PDF files that look similar on a computer screen may be of very different sizes. For example, a high resolution raster image takes more space than a low resolution one. Typically higher resolution is needed for printing documents than for displaying them on screen. Other things that may increase the size of a file is embedding full fonts, especially for Asiatic scripts, and storing text as graphics.

Standard Type 1 Fonts

There are fourteen typefaces that have a special significance to PDF documents:

• Times (v3) (in regular, italic, bold, and bold italic)
• Courier (in regular, oblique, bold and bold oblique)
• Helvetica (v3) (in regular, oblique, bold and bold oblique)
• Symbol
• Zapf Dingbats

These fonts, sometimes referred to as the «base fourteen fonts»^[55] should always be present (actually present or a close substitute) and so need not be embedded in a PDF.^[56] PDF viewers must know about the metrics of these fonts. Other fonts may be substituted if they are not embedded in a PDF.

Versions

Version	Year of publication	New features	Supported version
1.0	1993		Acrobat Reader (Carousel)
1.1	1996	Passwords, encryption (MD5, RC4 40bit), device-independent color, threads and links[57]	Acrobat Reader 2.0
1.2	1996	Interactive page elements (such as radio buttons and checkboxes); interactive, fill-in forms (AcroForm); Forms Data Format (FDF) for interactive form data that can be imported, exported, transmitted and received from the Web; mouse events; support for playing movies from external files; support for playing sounds, either embedded in the PDF file or from external files ; Unicode; advanced color features and image proxying[57]	Acrobat Reader 3.0
1.3	2000	Digital signatures; ICC and DeviceN color spaces; JavaScript actions; Embedded file streams — embedding files of any type directly within the body of the PDF file itself (e.g. used for attachments); many new annotation types; new features of the Adobe imaging model embodied in PostScript LanguageLevel 3; Masked images; Alternate representations for a single image; Smooth shading; Enhanced page numbering; Web Capture — a facility for capturing information from World Wide Web and converting it to PDF; Logical structure — a facility for representating logical structure independently of its graphical structure; additional support for CIDFonts; data structures for mapping strings and numbers to PDF objects; Prepress Support — information useful in prepress production workflows; new functions — for several types of function object that represent parameterized classes of functions[58][59]	Acrobat Reader 4.0
1.4	2001	JBIG2; transparency; RC4 encryption key lengths greater than 40bits (40-128bits); enhancements to interactive forms and Forms Data Format (FDF), support for XML form submissions, embedded FDF files, Unicode specification of field export values, support for remote collaboration and digital signatures in FDF files; support for accessibility to disabled users; metadata streams using XML — Extensible Metadata Platform (XMP); Tagged PDF ;Facilities for including printer’s marks; Support for the display and preview of production-related page boundaries; New predefined CMaps; Alternate Presentations — alternate ways in which the document may be viewed; The ability to import content from one PDF document into another; EmbeddedFiles entry in the PDF document’s name dictionary — a standard location for the embedded data[59][60]; OCR text layer[citation needed] || Acrobat Reader 5.0
1.5	2003	JPEG 2000; enhanced support for embedding and playback of multimedia; object streams; cross reference streams; XML Forms Data Format (XFDF) for interactive form submission (replaced the XML format in PDF 1.4); support for forms, rich text elements and attributes based on Adobe’s XML Forms Architecture (XFA) 2.02; public-key security handlers using PKCS#7 (introduced in PDF 1.3 but not documented in the PDF Reference until PDF 1.5), public-key encryption, permissions — usage rights (UR) signatures (does not require document encryption), PKCS#7 with SHA-1, RSA up to 4096-bits; security handler can use its own encryption and decryption algorithms; Optional Content — sections of content in a PDF document that can be selectively viewed or hidden by document authors or consumers — for items such as CAD drawings, layered artwork, maps, and multi-language documents; Alternate Presentations — the only type is slideshow — invoked by means of JavaScript actions (Adobe Reader supports only SVG 1.0)[59][61][62] Support for Windows 98 dropped. To view and print newer version PDFs, such as those at the IRS website, with older versions of Reader requires downloading in Google Docs «Quick View» simplified PDF format.	Adobe Reader 6.0
1.6	2004	3D artwork, e.g. support for Universal 3D file format; OpenType font embedding; support for XFA 2.2 rich text elements and attributes; AES encryption; PKCS#7 with SHA256, DSA up to 4096-bits; NChannel color spaces; additional support for embedded file attachments, including cross-document linking to and from embedded files; enhancements and clarifications to digital signatures related to usage rights and modification detection and prevention signatures[59]	Adobe Reader 7.0
1.7	2006	increased presentation of 3D artwork; support for XFA 2.4 rich text elements and attributes; presentation of multiple file attachments (portable collections); document requirements for a PDF consumer application; new string types — PDFDocEncoded string, ASCII string, byte string; PKCS#7 with SHA384, SHA512 and RIPEMD160	Adobe Reader 8
1.7 Extension Level 3	2008	256-bit AES encryption; incorporation of XFA Datasets into a file conforming PDF/A-2; RichMedia annotations — attaching Flash applications, video (including Flash video with H.264), audio, and other multimedia with expanded functionality, two-way scripting bridge between Flash and a conforming application; support for the rich text conventions described in XFA 2.5 and 2.6	Adobe Reader 9
1.7 Extension Level 5	2009	XFA 3.0	Adobe Reader 9.1

Implementations

PDF-viewing software is generally provided free of charge, including versions by Adobe Reader, Foxit Reader, PDF-XChange Viewer, Sorax Reader and others.

There are many software options for creating PDFs, including the PDF printing capabilities built in to Mac OS X and some versions of Linux, the multi-platform OpenOffice.org, Microsoft Office 2007 (if updated to SP2),^[63] WordPerfect since version 9, Free PDF XP ^[64] and numerous PDF print drivers for Microsoft Windows, the pdfTeX typesetting system, the DocBook PDF tools, applications developed around Ghostscript and Adobe Acrobat itself. Google’s online office suite Google Docs also allows for uploading, and saving to the PDF format.

Editing PDFs (structure)

There is also specialized software for editing PDF files, though the choices are much more limited and often expensive. As of version 0.46, Inkscape also allows PDF editing through an intermediate translation step involving Poppler.

Annotating PDFs

See also: Comparison of notetaking software and Category:Web annotation

Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, add notes to already created PDF files. One UNIX application available as free software (under the GNU General Public License) is PDFedit. Another GPL-licensed application native to the Linux environment is Xournal. Xournal allows for annotating in different fonts and colours, as well as a rule for quickly underlining and highlighting lines of text or paragraphs. Xournal also has a shape recognition tool for squares, rectangles and circles. In Xournal annotations may be moved, copied and pasted. The freeware Foxit Reader allows annotating but adds a watermark on each annotated page. The commercial version of the package does not have this limitation. Tracker Software’s PDF-XChange Viewer allows annotations and markups without restrictions in its freeware alternative. Apple’s Mac OS X’s integrated PDF viewer, Preview, does also enable annotations. For mobile annotation, iAnnotate PDF for the iPad and Aji Annotate for the iPhone, both produced by Aji, allow annotation of PDFs as well as exporting summaries of the annotations.

There are also web annotation systems which allow to annotate pdf and other documents formats, e.g. A.nnotate, crocodoc, WebNotes.

Other applications and functionalities

Several applications embracing the PDF standard are now available as an online service including Scribd for viewing and storing, Pdfvue for online editing, and Zamzar for PDF Conversion.

In 1993 the Jaws RIP from Global Graphics became the first shipping prepress RIP that interpreted PDF natively without conversion to another format. The company released an upgrade to their Harlequin RIP with the same capability in 1997.^{[citation needed]}

Agfa-Gevaert introduced and shipped Apogee, the first prepress workflow system based on PDF, in 1997.

Many commercial offset printers have accepted the submission of press-ready PDF files as a print source, specifically the PDF/X-1a subset and variations of the same.^[65] The submission of press-ready PDF files are a replacement for the problematic need for receiving collected native working files.

PDF was selected as the «native» metafile format for Mac OS X, replacing the PICT format of the earlier Mac OS. The imaging model of the Quartz graphics layer is based on the model common to Display PostScript and PDF, leading to the nickname «Display PDF». The Preview application can display PDF files, as can version 2.0 and later of the Safari web browser. System-level support for PDF allows Mac OS X applications to create PDF documents automatically, provided they support the Print command. The files are then exported in PDF 1.3 format according to the file header. When taking a screenshot under Mac OS X versions 10.0 through 10.3, the image was also captured as a PDF; in 10.4 and 10.5 the default behaviour is set to capture as a PNG file, though this behaviour can be set back to PDF if required.

Some desktop printers also support direct PDF printing, which can interpret PDF data without external help. Currently, all PDF capable printers also support PostScript, but most PostScript printers do not support direct PDF printing.

The Free Software Foundation considers one of their high priority projects to be «developing a free, high-quality and fully functional set of libraries and programs that implement the PDF file format and associated technologies to the ISO 32000 standard.»^[66][67] The GNUpdf library has, however, not been released yet, while Poppler has enjoyed wider use in applications such as Evince, which comes with the GNOME desktop environment, at the expense of relying on the GPLv2-licensed Xpdf^[68][69] code base that can’t be used by GPLv3 programs.

The Apache PDFBox project of the Apache Software Foundation is an open source Java library for working with PDF documents. PDFBox is licensed under the Apache License.^[70]

References

External links

• Edit or make changes to a PDF file at Microsoft Support
  • Add a PDF to your Office file
  • Save PowerPoint presentations as PDF files
  • Save As or convert a publication to .pdf or .xps using Publisher
  • Word Help & Training: Edit a PDF
  • PDF reader in Microsoft Edge at Microsoft Learn
  • Adobe Acrobat for Microsoft 365
• Adobe: PostScript vs. PDF – Official introductory comparison of PS, EPS vs. PDF^{[dead link]}
  • Adobe PDF 101: Quick overview of PDF^{[dead link]}
  • Latest PDF version specification (including free version of the ISO standard)
  • PDF Reference Books: PDF 1.6 (ISBN 0-321-30474-8), PDF 1.4 (ISBN 0-201-75839-3), PDF 1.3 (ISBN 0-201-61588-6)
• ISO 19005-1:2005 Document of the PDF/A-1 Standard at the International Organization for Standardization (chargeable)
• Portable Document Format: An Introduction for Programmers – Quick one-page intro to PDF vs. PostScript and PDF internals (up to v1.3)
• Free PDF files hosting service
• Portable Document Format at Wikipedia

Portable Document Format (PDF) — это формат файлов, разработанный Adobe в 1993 году для представления документов, включая форматирование текста и изображения, независимо от прикладного программного обеспечения, оборудования и операционных систем.^[1][2] На основе языка PostScript каждый файл PDF инкапсулирует полное описание плоского документа с фиксированным макетом, включая текст, шрифты, векторную графику, растровые изображения и другую информацию, необходимую для его отображения.

PDF был стандартизирован как ISO 32000 в 2008 году, и больше не требует никаких лицензионных отчислений за его внедрение.^[3]

Файлы PDF могут содержать различное содержимое помимо простого текста и графики, включая элементы логической структуризации, интерактивные элементы, такие как аннотации и поля формы, слои, мультимедийные материалы (включая видеоконтент) и трехмерные объекты, использующие U3D или PRC, а также различные другие форматы данных. Спецификация PDF также предусматривает шифрование и цифровые подписи, вложения файлов и метаданные, чтобы обеспечить рабочие процессы, требующие этих функций.

История[править]

Adobe Systems предоставила спецификацию PDF бесплатно в 1993 году. В первые годы PDF был популярен в основном в рабочих процессах настольных издательских систем и конкурировал с различными форматами, такими как DjVu, Envoy, Common Ground Digital Paper, Farallon Replica и даже собственным форматом Adobe PostScript.

PDF был проприетарным форматом, контролируемым Adobe, пока он не был выпущен в качестве открытого стандарта 1 июля 2008 года и опубликован Международной организацией по стандартизации как ISO 32000-1: 2008,^[4][5] в то время как спецификация передана комитету ISO, состоящему из добровольных отраслевых экспертов. В 2008 году Adobe опубликовала публичную патентную лицензию ISO 32000-1, предоставляющую бесплатные права на все патенты, принадлежащие Adobe, которые необходимы для создания, использования, продажи и распространения реализаций, совместимых с PDF.^[6]

PDF 1.7, шестое издание спецификации PDF, ставшее стандартом ISO 32000-1, включает некоторые проприетарные технологии, определенные только Adobe, такие как Adobe XML Forms Architecture (XFA) и расширение JavaScript для Acrobat, на которые в ISO 32000-1 ссылаются как нормативный и необходимый для полной реализации спецификации ISO 32000-1. Эти проприетарные технологии не стандартизированы, и их спецификации публикуются только на веб-сайте Adobe.^{[7][8][9][10][11]} Многие из них также не поддерживаются популярными сторонними реализациями PDF.

28 июля 2017 года был опубликован стандарт ISO 32000-2: 2017 (PDF 2.0)^[12]. ISO 32000-2 не содержит каких-либо патентованных технологий в качестве нормативных ссылок.^[13]

Технические основы[править]

Файл PDF часто представляет собой комбинацию векторной графики, текста и растровой графики. Основные типы содержимого PDF-файла:

• Текст, хранящийся в виде потоков контента (то есть не закодированный в виде обычного текста)
• Векторная графика для иллюстраций и дизайнов, состоящих из форм и линий.
• Растровая графика для фотографий и других типов изображений
• Мультимедийные объекты в документе

В более поздних версиях PDF документ PDF может также поддерживать ссылки (внутри документа или веб-страницы), формы, JavaScript (изначально доступный как плагин для Acrobat 3.0) или любые другие типы встроенного содержимого, которые можно обрабатывать с помощью плагинов.

PDF объединяет три технологии:

• Подмножество языка программирования описания страниц PostScript для создания макета и графики.
• Система встраивания/замены шрифтов, позволяющая шрифтам перемещаться вместе с документами.
• Структурированная система хранения для объединения этих элементов и любого связанного с ними содержимого в один файл со сжатием данных, где это необходимо.

PostScript — это язык описания страниц, запускаемый в интерпретаторе для создания изображения, процесс, требующий много ресурсов. Он может обрабатывать графику и стандартные функции языков программирования, такие как операторы if и команды цикла. PDF в значительной степени основан на PostScript, но упрощен, чтобы удалить подобные функции управления потоком, в то время как графические команды, такие как lin, остаются.

Часто код PDF, подобный PostScript, создается из исходного файла PostScript. Графические команды, выводимые кодом PostScript, собираются и токенизируются. Также собираются все файлы, изображения или шрифты, на которые ссылается документ. Затем все сжимается в один файл. Таким образом, весь мир PostScript (шрифты, макет, размеры) остается нетронутым.

Как формат документа PDF имеет несколько преимуществ перед PostScript:

• PDF содержит токенизированные и интерпретированные результаты исходного кода PostScript для прямого соответствия между изменениями элементов в описании страницы PDF и изменениями внешнего вида страницы.
• PDF (с версии 1.4) поддерживает прозрачную графику; PostScript — нет.
• PostScript — это интерпретируемый язык программирования с неявным глобальным состоянием, поэтому инструкции, сопровождающие описание одной страницы, могут повлиять на внешний вид любой следующей страницы. Следовательно, все предыдущие страницы в документе PostScript должны быть обработаны, чтобы определить правильный внешний вид данной страницы, в то время как каждая страница в документе PDF не зависит от других. В результате средства просмотра PDF позволяют пользователю быстро переходить к последним страницам длинного документа, в то время как средство просмотра PostScript должно обрабатывать все страницы последовательно, прежде чем сможет отобразить целевую страницу (если дополнительные соглашения о структурировании документов PostScript не были тщательно скомпилированы и включены).

PDF 1.6 поддерживает интерактивные 3D-документы, встроенные в файл PDF: 3D-чертежи могут быть встроены с использованием U3D или PRC и различных других форматов данных.

Источники[править]

Ссылки[править]

• PDF Specifications, including the PDF Reference for PDF 1.7, PDF 1.6 (ISBN 0-321-30474-8), PDF 1.5, PDF 1.4 (ISBN 0-201-75839-3), PDF 1.3 (ISBN 0-201-61588-6)

Медиаконтейнеры [+]
Видео/аудио	3GP • ASF • AVI • Bink • DivX_Media_Format#DivX_Media_Format_.28DMF.29 • DPX • Enhanced_VOB • FLV • Matroska (MKV) • WebM • MPEG-PS • MPEG-TS • MP4 • MXF • NUT • Ogg • Ogg Media • QuickTime • RealMedia • Smacker • RIFF • VOB • сравнение • сжатие
Аудио	AIFF • APE • AU • DSD • DXD • MLP • MP3 • FLAC • SHN • WAV • WMA • сравнение • сжатие
Графические форматы (сжатие)
Растровые	Без потерь: BMP • FPX • GIF • ICO • ILBM • JBIG • PCX • PNG • PNM • PSD • Raw • TGA • WBMP • XCF • Включая сжатие с потерями: BPG • EXR • ICER • JBIG2 • JPEG / JP2 / JPEG-LS • JPEG XR (HD Photo) • PGF • TIFF • WebP • Анимационные: APNG • GIF • MNG
Векторные	AI • CDR • EMF • EPS • PS • SVG • WMF • XPS • Анимационные: SVG • SWF • 3D: 3DS • VRML • X3D
Комплексные	CGM • DjVu • PDF

Adobe Systems [+]
Клиентское ПО	Creative Cloud (Creative Suite) · Technical Communication Suite · Acrobat · Acrobat Connect · Audition · Captivate · Digital Editions · Director · GoLive · PageMaker · Photoshop Lightroom · FreeHand · Media Encoder CC · больше
Серверное ПО	ColdFusion · LiveCycle · Flash Media Server · JRun · Premiere Express · Photoshop Express
Технологии	PostScript · PDF · FlashPaper · Authorware · Flash · Font Folio · DNG · Flex · AIR · BlazeDS
Сервисы	Adobe Solutions Network
Совет директоров	Charles Geschke · John Warnock · Bruce Chizen · Shantanu Narayen
Приобретение других компаний	Объединения и приобретения · Aldus · Macromedia · Scene7
Категория · Викисклад

Электронные книги (устройства и документы) [+]
Устройства	Amazon Kindle • Nook • FR book • LBook • MAGIC E701 • Onext Touch&Read • Onyx Boox • PAGEone • PocketBook Reader • REB 1100 • Rocket eBook • Sony Reader • Азбука • Электронная книга с шрифтом Брайля • список…
Форматы файлов	CBR • DjVu • DOC • ePub • FB • ODF • PDF • BBeB • TXT
Каталогизаторы	All My Books • Calibre • MyHomeLib • MyRuLib
Библиотеки	Amazon.com • Barnes & Noble • FictionBook.ru • Google Books • MyBook • Ozon.ru • Альдебаран • Архив Интернета • Библиотека Мошкова • Викитека • Либрусек • ЛитРес • Марксистский интернет-архив • Проект «Гутенберг» • Руниверс • Флибуста
См. также	Электронная бумага