PDFs are widely used to provide electronic forms of printed documents. With a little care they can be fully accessible.
There are a number of simple tools you can use to check the accessibility of existing PDFs see:
If your starting point is an accessible Word document then it is easy to generate an accessible PDF using Adobe Acrobat or Word 2007. See:
If PDFs are badly structured, or have an incorrect reading order then these issues can be corrected using Adobe Acrobat Professional.