phranque - 12:43 pm on Aug 30, 2012 (gmt 0)
here are some relatively useless (for your question) resources from google on pdfs.
Official Google Webmaster Central Blog: PDFs in Google search results:
What are the best practices for PDF optimization? - Matt Cutts YouTube video:
the TLDR version is you should set the meta title properly and use text rather than images.
you will not see any type of meta description unless you remove the robots crawling exclusion for the pdfs.
typically i see the first part of the text in the document as the description.
i would suggest looking for cases where pdf metadata is used in snippets by doing some research with a filetype:pdf Google Search: