A Google Webmaster Help thread has a nice little crawling tidbit for advanced SEOs. Did you know that it typically takes a long time for a PDF document to be recrawled and indexed by Google? Exactly how long, I am not sure, I wonder if someone has done some tests on that.

But JohnMu from Google made a comment about this topic, saying:

So when you have PDF documents on the web, keep in mind, do it right or it can take a while for those PDFs to be recrawled.

Forum discussion at Google Webmaster Help.