About a month ago, a government legislator asked me about PDF and accessibility. I told him what I knew about tagging these files with structural information, made a mental note to do a blog entry about it, and then promptly forgot about it.
Luckily for me and you, Duff Johnson has done a very nice blog entry that explains all of this. Here’s a little bit from his “Blind Spots: PDF and Section 508 compliance“:
Beyond the simplest of documents, what is required in order to be considered “accessible”, document contents must be structured such that tables, images, footnotes and so on are correctly identified to the user. Without structure, documents are just a heap of words – or letters, if you prefer to assume away the structure that binds the letters and words together into paragraphs.
Sighted users get to “cheat” by gaining clues about tables, lists and so on from the page layout. Those who must use alternative reading methods to read the same document often can�t access this presentational information. They are dependant on either the structure (or lack thereof) in the document, or else the capacity of their reading software to correctly impute the structure based on a programmatic examination of the page.