This is, by far, the most common plug-in based content. The publisher of Acrobat (to create PDF documents), Adobe, has a document to describe how to make PDF documents accessible. You can find this document at http://www.adobe.com/enterprise/accessibility/pdfs/acro7_pg_ue.pdf.
It suffices to say that it is a bit out of the scope of a web accessibility class to cover a 115 document on PDF document accessibility. However, it is important to note that Adobe emphasizes that documents need to be created with accessibility in mind in the first place, and not as an afterthought or fix.
Note that Acrobat reader itself has accessibility features. As a result, no additional plug-in is required for tool-wise accessibility. The main problem remains to be with the content of a document. Essentially, if a PDF document is to be more accessible, someone needs to manaully go through the document, and add necessary tags to improve accessibility.