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What Is An Accessible PDF?

WAC Workshop. January 2005. Written and Presented by: Lori Bailey.

Covers: Levels of Accessibility, Benefits of Tagged PDF, Requirements for 508 Compliant Accessible PDF, Additional (Recommended) Standards.

Accessibility Levels for PDF Documents

Before considering how to prepare or retrofit a PDF document for accessibility, you must understand the issues within a non- or semi-accessible PDF document.

Three levels of PDF Accessibility

1. Not Structured: Document is completely inaccessible. Screen-readers not able to read the text: no tags, text is not selectable or sizable, not able to reflow the page, and images not identified through ALT-text.
2. Structured: Document is somewhat accessible (best for documents without complex structure such as columns, data tables, footers, and side bars). Document has no tags, not able to reflow, images have no ALT-text, table column and row headers aren't defined, and screen-readers may read text out-of-order, skip or incorrectly interpret sections of text, or read tables across rather than by cell.
3. Tagged: Document is fully accessible to software that supports PDF interpolation (Jaws, Window Eyes, Hal). Document tags use many elements of the tag structure to identify sections, divisions, captions, tables, images, and other special elements. Tables are fully rendered using tags to identify column and row headers and data content. Read-order is identified throughout document.

Benefits of Tagged PDF

Creating a tagged PDF document will resolve a majority of the accessibility issues with PDFs, and, for complex documents containing columns and extensive data tables, may offer the only method for meeting Section 508 accessibility standards. Tagged PDF represents various components of a document, such as chapters, heading styles, blocks of text, tables, graphics, and so on, as tag elements.

• The tag structure is similar to markup languages such as HTML and XML. A document's structure is represented as a hierarchy of tag elements. The order in the hierarchy represents the reading order of the document. Since the content is represented with tag elements, other applications can extract the information and reuse it for other purposes. Tagged PDF offers the following benefits:
• You can associate additional information with a particular tag element, such as a graphic, by filling in its alternate text property.
• Other applications can automatically reflow text and associated graphics to fit a page of a different size than was assumed for the original layout.
• The document's content can be converted to other common file formants (such as RTF, HTML, and XML) while preserving the structure and basic style information.
• Provides usability enhancements, including enhanced keyboard shortcuts, support for high-contrast viewing, and the ability to zoom in and reflow text on the screen.
• Supports screen readers: Provides direct support for screen readers via the Microsoft Active Accessibility (MSAA) application programming interface (API) for Windows.® MSAA enables Acrobat 5.0 to integrate with assistive technology products including the newest versions of screen readers from vendors such as Freedom Scientific (http://www.freedomscientific.com), GWMicro (http://www.gwmicro.com), and Dolphin Oceanic (http://www.dolphinoceanic.com).

Requirements for 508 Compliant Accessible PDF

A fully accessible PDF will address or provide work-arounds for these issues:

• Read-order: Encoding the proper, or logical, reading order for a document is one of the most fundamental steps towards accessibility. This is especially critical for documents with columnated formats or where there are several distinct text blocks. For example, if a document has been correctly authored using two columns to create a two-column format, the screen reader knows it should read all the way down the first column and then proceed to the second column. On the other hand, if the writer used tabs to imitate the look of two-column text, the screen reader would not recognize the layout as two-column. Instead, it would simply read horizontally, going from the first line in the first column and then tabbing over to the first line in the second column. To insure that your graphics retain their proper reading order, you must also make sure that the graphic has been inserted inline with the text.
  Structured PDF text is presented in single column only.
  Tagged PDF Header-levels convert to tags and tag order indicates read-order.
• Reflow: Ability to reformat, or reflow, document content. Magnifying the font size is not a viable solution since the viewer must scroll widely in all directions to cover the document. Instead, an application should reflow the magnified text into the available screen space such that the text reformats itself to conform to that space. Text reflow is also used, for example, to reformat a document for display on a Palm Pilot, although that is not a primary concern for accessibility. Text reflow is a good example of a feature that benefits users who do not have disabilities.
  Structured PDF and Tagged PDF use only TrueType fonts and all scanned or imaged text has been converted via "OCR", so it is selectable and sizable.
• Alternate text for images: include alternative, descriptive, text for all images. For example, in Acrobat, you can use the Tags palette to add a description of an image. Then, when a screen reader encounters that image in the document, it will read the alternate text description so the user can understand what the image is about.
  You should seriously evaluate the information that a visual is conveying. Is the information already present in the text? Is the visual simply providing color and images that are not essential to the message conveyed by the document? The alternate text that you tie to a visual should not repeat the caption.
  Structured PDF include descriptions of all images in the body of the text. Example: "Image 5.1 depicts a young boy on a swing."
  Tagged PDF use originating software to add "ALT tags" to images; when converted to PDF, images maintain "tags." This allows for more detailed description than might be used in the body text.
• Alternate descriptions for data tables: add descriptive text for data tables. Do not "nest" data tables by putting one table inside another. Use a distinctive cell for each data entry.
  Structured PDF add descriptive text within the body of the text either immediately above or below the table.
  Tagged PDF add descriptive text using table markup in the originating software; identify column and row headers and include descriptive summaries.
• Disable/limit document security settings: A document's security settings will make it inaccessible to screen readers if it's an Acrobat 4.0-compatible document and copying text and graphics is prohibited, or if it's an Acrobat 5.0-compatible document and accessibility is prohibited

Additional Recommended Standards

• Provide document navigation using bookmarks (also called anchors or in-document links): Users of screen-readers have fewer options for moving through a document. Especially in longer documents, like dissertations, in-document links can facilitate getting to particular portions of the document quickly. Minimum standard: set PDF conversion so that each header tag is also converted into a bookmark. Best practice (in addition to the minimum standard):
  • add in-document links to the table of contents and/or document index,
  • add in-document links to all data tables, relevant images, and side-bars or notes,
  • add "return to contents" (or navigation page) link to the footer (end) of each section or page.
  • Note: For documents created in supported software with correct conversion settings, heading bookmarks will match the entries for Table of Contents, unless stylesheets were not used properly to identify the start of each chapter/section. However, adding in-document links to the table of contents allows the user to jump to various sections of the document without moving between the bookmark and document windows in Adobe Acrobat Reader.
• Specify a document language and any instances of changes from the primary language: Although current screen readers are designed to read the document according to one particular language, specifying the language of the document in the structure can benefit future screen reader technologies that may be able to change languages on the fly.
• Use Adobe's "Accessibility Checker" to identify and repair any unresolved accessibility issues: After creating your PDF document, Acrobat provides a tool called the Accessibility Checker to check various aspects of accessibility for a document. Tip: if you turn on the "comment creation" before running the checker, you can page through the document and note where the Accessibility Checker has flagged problems in the document.

Portions of this document adapted from "Authoring for Accessibility and Reflow in Microsoft Word and Adobe Acrobat," an Adobe Acrobat tutorial.

This document was created with the Illinois Accessible Web Publishing Wizard for Office.