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:
Reprocessing a PDF can generally take a bit of time, so if it is urgent that you have this information removed from all places, it would be necessary to do a normal URL removal. One way to do this would be to do as Squibble suggests, simply renaming the PDF file so that you can request that the old URL be removed. Apart from that, there is no way to speed up these processes for a single URL.
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.