as I mentioned recently, in recent meetings with existing Acrobat users, I've been answering quite a few questions regarding accessibility. My initial answer has always been that PDF is the best place to start to create accessible documents, but why is that? Well PDF files have certain characteristics that can make them more accessible.......
Accessible PDFs have the following characteristics.
A document that consists of scanned images of text is inherently inaccessible because the content of the document is images, not searchable text. Assistive software cannot read or extract the words, users cannot select or edit the text, and you cannot manipulate the PDF for accessibility. Convert the scanned images of text to searchable text using optical character recognition (OCR) before you can use other accessibility features with the document.
Alternate text descriptions
Screen readers cannot read document features such as images and interactive form fields unless they have associated alternate text. Screen readers can read web links; however, you can provide more meaningful descriptions as alternate text. Alternate text and tool tips can aid many users, including users with learning disabilities.
Fonts that allow characters to be extracted to text
The fonts in an accessible PDF must contain enough information for Acrobat to extract all of the characters to text for purposes other than displaying text on the screen. Acrobat extracts characters to Unicode text when you read a PDF with a screen reader or the Read Out Loud feature. Acrobat also extracts characters to Unicode when you save as text for a braille printer. This extraction fails if Acrobat cannot determine how to map the font to Unicode characters.
Reading order and document structure tags
To read a document's text and present it in a way that makes sense to the user, a screen reader or other text-to-speech tool requires a structured document. Document structure tags in a PDF define the reading order and identify headings, paragraphs, sections, tables, and other page elements.
Interactive form fields
Some PDFs contain forms that a person is to fill out using a computer. To be accessible, form fields must be interactive—meaning that a user must be able to enter values into the form fields.
Navigational aids in a PDF—such as links, bookmarks, headings, a table of contents, and a preset tab order for form fields—assist all users in understanding the document without reading completely through it. Bookmarks are especially useful and can be created from document headings.
Specifying the document language in a PDF enables some screen readers to switch to the appropriate language.
Security that doesn't interfere with assistive software
Some authors of PDFs restrict users from printing, copying, extracting, adding comments to, or editing text. The text of an accessible PDF must be available to a screen reader. You can use Acrobat to ensure that security settings don't interfere with the ability of the screen reader to convert the on-screen text to speech.