I was at a session by Maurice Prather on Secure Coding for the Administrator at the Australian SharePoint Conference 2011. It was a good session and there was a mention of the risks of PDF handling in the browser.
I did some more reading and found that Maurice had written an article which explains the risks of PDF (Pretty Dangerous Files) well.
I thought this will be useful to make note of it in my blog. Here is a link to Maurice Prathers article on PDF (Pretty Dangerous Files):