In the US sites should be made accessible for people with disabilities. This is often followed for HTML, but ignored for PDF files. Since there is quite a lot of PDF content on the web, what tools do you use to make that content accessible?
http://section508.gov/ has the facts for the United States.
We use an Adobe Acrobat Pro plugin called CommonLook
http://www.net-centric.com/products/cl_s508_adobe.aspx
It is pretty slow on big documents, has some real navigation issues, but does make editing the tags and content of a PDF much easier than the default tools in Adobe Acrobat 9 Pro.
PDF can be accessible as it is it depends on the reader, generally speaking if a person with a visual impairment is opening PDFs they will have the appropriate software.
If the content you are providing is just text and you want to be ultra accessible then you should consider allowing the same file to be downloaded as a .txt file - .txt can be read by any machine with no programs to install and will be picked up by screen reading software.
Of course this only covers visual impairment, but most other accessibility concerns are to do with content, in which case you should follow the same advice as you would when writing content meant to be marked up with HTML.