Adobe Acrobat and PDF
http://ncdae.org/tools/factsheets/pdf.cfm

NCDAE
2006

Abstract:

After HTML, PDF (Portable Document Format) files are probably the most common files on the Web. PDF is usually used when a file needs to appear or print a certain way, regardless of the browser or technology. PDF files can be made accessible to people with disabilities, although usually with more difficulty than with HTML. A key part of this process involves creating tags that make a document more accessible to screen reader users.