PDF was originally designed to do one thing: deliver an author’s intent to screen or printer in an efficient, precise and platform-independent manner. This “print paradigm” persists today, and colors the issue of accessible PDF in subtle but pervasive ways.
To understand how the print orientation in PDF presents a challenge to accessibility, begin with the fact that a PDF has no intrinsic notion of words or paragraphs.
PDF “thinks” like a printer “thinks”. Objects (such as text, images, lines, etc.) appearing on the page are considered in terms of precise location and sequence of appearance, not their semantic relationship to one another. Words and paragraphs aren’t useful constructs in printing, for they offer no assistance in the placement of ink. As a consequence, the very concept of a word was literally missing from PDF until 2000, when Adobe published the basic mechanism for “structured” PDF with the PDF Reference 1.3.
What is structured PDF? Simply put, it is PDF with additional information to organize the objects on the page into words, lines and paragraphs, and to order these larger blocks of content into a “text flow”.
In late 2001, Adobe published the PDF Reference 1.4, which described “tagged” pdf, adding organization and nomenclature to the structure elements.
Tags allow the author to manage text-flow, define headings, add alternate text to images, ensure tables match the intent, and generally ensure that the document contents are fully and discreetly available to users who can benefit from such information.
Who are these consumers of of structured and tagged PDF? They include disabled and other AT (assistive technology) users, but anyone trying to copy-and-paste text or read a PDF on a mobile device will prefer tagged to untagged PDF.
Beyond strictly human “users”, tagged PDF improves search engine results, and even makes possible the extension of semantic Web concepts to PDF.
Those interested in more specific or technical information should check out Mark Gavin’s presentation, PDF and Accessibility, at labs.appligent.com.
In a forthcoming post, I’ll lay out the business case for better tools to enhance PDF accessibility.
by Duff Johnson