User Agent Support Notes for PDF Techniques

Logical Structure

PDF's logical structure features (introduced in PDF 1.3) provide a mechanism for incorporating structural information about a document's content into a PDF file. Such information might include, for example, the organization of the document into chapters, headings, paragraphs and sections or the identification of special elements such as figures, tables, and footnotes. The logical structure features are extensible, allowing applications that produce PDF files to choose what structural information to include and how to represent it, while enabling PDF consumers to navigate a file without knowing the producer's structural conventions.

PDF logical structure shares basic features with standard document markup languages such as HTML, SGML, and XML. A document's logical structure is expressed as a hierarchy of structure elements, each represented by a dictionary object. Like their counterparts in other markup languages, PDF structure elements can have content and attributes. In PDF, rendered document content takes over the role occupied by text in HTML, SGML, and XML.

A PDF document's logical structure is stored separately from its visible content, with pointers from each to the other. This separation allows the ordering and nesting of logical elements to be entirely independent of the order and location of graphics objects on the document's pages.

The logical structure of a document is described by a hierarchy of objects called the structure hierarchy or structure tree. At the root of the hierarchy is a dictionary object called the structure tree root, located by means of the StructTreeRoot entry in the document catalog. See Section 14.7.2, ("Structure Hierarchy") in PDF 1.7 (ISO 32000-1): Table 322 shows the entries in the structure tree root dictionary. The K entry specifies the immediate children of the structure tree root, which are structure elements.

Tagged PDF

Tagged PDF (PDF 1.4) is a stylized use of PDF that builds on PDF's logical structure framework. It 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. It is intended for use by tools that perform the following types of operations:

• Simple extraction of text and graphics for pasting into other applications.

• Automatic reflow of text and associated graphics to fit a page of a different size than was assumed for the original layout.

• Processing text for such purposes as searching, indexing, and spell-checking.

• Conversion to other common file formats (such as HTML, XML, and RTF) with document structure and basic styling information preserved.

• Making content accessible to people who rely on assistive technology.

Generating PDF Files

Many applications can generate PDF files directly, and some can import them as well. This direct approach is preferable, since it gives the application access to the full capabilities of PDF, including the imaging model and the interactive and document interchange features. Alternatively, applications that do not generate PDF directly can produce PDF output indirectly. There are two principal indirect methods:

• The application describes its printable output by making calls to an application programming interface (API) such as GDI in Microsoft® Windows® or QuickDraw in the Apple Mac OS. A software component called a printer driver intercepts these calls and interprets them to generate output in PDF form.

• The application produces printable output directly in some other file format, such as PostScript, PCL, HPGL, or DVI, which is converted to PDF by a separate translation program.

Although these indirect strategies are often the easiest way to obtain PDF output from an existing application, the resulting PDF files may not make the best use of the high-level PDF imaging model relied upon to expose the semantics of the document. This is because the information embodied in the application's API calls or in the intermediate output file often describes the desired results at too low a level. Any higher-level information maintained by the original application has been lost and is not available to the printer driver or translator.

For example, since the printer driver or translator targets correct visual output, information about the semantics of the document may not be sent at all or may be ignored when creating the PDF output. As a result, headings may not be tagged as such, or link text may not be associated with its link object. Check with the vendor of any PDF authoring tool in order to understand how to use the tool in a way that produces the best tagged output.

PDF Authoring Tools that Provide Accessibility Support

• Adobe Acrobat's PDFMaker - PDFMaker is part of Adobe Acrobat which adds macros to many business applications such as Microsoft Office, AutoCAD and Lotus Notes that support the conversion of content from the original format to tagged PDF.

• Adobe FrameMaker - Desktop publishing application from Adobe Systems that directly exports tagged PDF and provides support for alternative text descriptions.

• Adobe InDesign - Page layout and desktop publishing application from Adobe Systems that directly exports tagged PDF and provides support for alternative text descriptions.

• Adobe LiveCycle Designer - Windows-based forms design application from Adobe Systems that directly exports tagged PDF interactive forms and provides support for alternative text descriptions; can be invoked standalone or from within Acrobat Pro.

• LibreOffice - Open-source word processing software from The Document Foundation that can export tagged PDF.

• Lotus Symphony Documents - Word-processing software from IBM that can export tagged PDF.

• Microsoft® Word - Word processing application from Microsoft Corporation that can export tagged PDF using the save as XPS or PDF utility.

• OpenOffice.org Writer - Open source word-processing software from Sun Microsystems Inc. that can export tagged PDF using the Export as PDF utility.

• CommonLook Office for Microsoft Office from Netcentric Technologies is an add-in to Microsoft® Word and PowerPoint that makes it possible to create tagged PDF documents. CommonLook Office provides tools to allow content authors to run accessibility tests in the Microsoft Word and PowerPoint environments and to remediate accessibility issues prior to conversion to PDF.

• Xenos Axess™ for Accessible Statements - PDF software integrates with an organization's existing enterprise content management (ECM) infrastructure to capture high-volume print streams and automatically transform them into tagged PDFs.

• WordPerfect® Office - Word-processing software from Corel that can be used to create, mark up, and share tagged PDF documents.

• Microsoft Office 10 - a suite of desktop office applications that creates tagged PDF.

Accessibility Checking and Repair

Adobe Acrobat Pro. Adobe Acrobat Pro is an application that creates and edits PDF files. It has a number of tools for evaluating and repairing the accessibility of PDF files, including access to the structure root through the tags panel, the ability to directly manipulate the reading order through the order panel, a built-in accessibility checker, and the Touch Up Reading Order tool which provides a graphical mechanism for assessing and repairing the accessibility of a PDF document.

Commonlook™ PDF. Commonlook PDF. Commonlook PDF is a plug-in for Adobe Acrobat Pro from Netcentric Technologies. CommonLook PDF helps identify, report and correct the most common accessibility problems, including the proper tagging of images, tables, forms and other non-textual objects.

PAC - the PDF Accessibility Checker. PAC is a free tool developed and distributed by the «Access for all» Foundation to evaluate the accessibility of PDF documents and PDF forms. PAC offers the added possibility of displaying a preview of the structured PDF document in a web browser. The PAC preview shows which tags are included in the PDF document and presents the accessible elements in the same way as they would be interpreted by assistive technologies (such as screen readers). PAC also provides an accessibility report which lists the detected accessibility errors. Clicking the links in the report displays the most probable source of the error within the document.

Adobe Reader and Acrobat Support for Accessibility APIs

Adobe provides methods to make the content of a PDF file available to assistive technology such as screen readers:

• On the Microsoft® Windows® operating system, Acrobat and Adobe Reader export PDF content as Component Object Model (COM) objects. Accessibility applications such as screen readers can interface with Acrobat or Adobe Reader in two ways:

  • Through the Microsoft Active Accessibility (MSAA) interface, using MSAA objects that Acrobat or Adobe Reader exports

  • Directly through exported COM objects that allow access to the PDF document's internal structure, called the Document Object Model (DOM).

• On UNIX® platforms, Adobe Reader supports the Gnome accessibility architecture. C-based Accessibility Toolkit (ATK) interfaces are available.

The DOM and MSAA models are related, and developers can use either or both. Acrobat issues notifications to accessibility clients about interesting events occurring in the PDF file window and responds to requests from such clients. Recent versions of Acrobat and Reader have enhanced the support for accessibility interfaces:

• MSAA interfaces are supported in Acrobat/Reader 5.0 and later.

• In Acrobat/Reader 6.0 and later, information about the underlying PDF structure is made available through direct COM objects that represent the PDF DOM. The DOM accessibility interfaces provide somewhat more extensive access.

• In Acrobat/Reader 7.0 and later, ATK and expanded DOM support is available.

• The Linux®, Solaris™, AIX®, and HP-UX versions of Adobe Reader implement C-based ATK interfaces, allowing screen readers, screen magnifiers, and on-screen keyboards to query an Accessibility Technology - Service Providers Interface (AT-SPI) registry for applications that are accessible.

• The DOM has been expanded to provide enhanced caret, selection, and focus support, as well as the new interfaces IPDDomDocument, ISelectText, and IPDDomNodeExt.

Assistive Technology Support

• JAWS 12 for Windows - screen reader from Freedom Scientific. Support for PDF started with JAWS version 4.

• MAGic 11 - screen magnifier from Freedom Scientific

• NVDA 2011.1 - NonVisual Desktop Access, open source screen reader distributed by NV Access. Providing feedback via synthetic speech and Braille, NVDA allows blind and vision-impaired people to access and interact with the Windows operating system and many third party applications.

• Supernova Access Suite 12.02 – full screen reader offering magnification, speech, and Braille support from Dolphin. Support for PDF started with HAL version 5.

• System Access To Go - screen reader from Serotek Corporation

• VoiceOver - screen reader for Mac OS X v10.6 Snow Leopard

• Window-Eyes 7.2 - screen reader from GW Micro. Window-Eyes was the first screen reader to provide support for PDF files, in Window-Eyes 4.2.

• ZoomText 9.1 - screen magnifier and screen reader from Ai Squared, with support for Adobe Acrobat and Reader:

  • PDF documents can be read using both AppReader and DocReader (without special settings)

  • PDF documents can be read in all Windows operating systems (without special settings)

  • AppReader and DocReader start instantly in Adobe Reader

  • PDF documents can be read with greater accuracy and without paging delays

  • PDF documents can be read in Internet Explorer (with the Adobe Reader plug-in)

  • Special Adobe Reader settings are no longer needed to obtain optimal reading