Google displays no cache date in their cached version of PDF files. Does somebody know any way to determine when a certain PDF file was cached?
According to a comment by Matt Cutts, former head of the web spam team at Google:
my guess is that it wouldn’t be trivial to add crawl dates to PDF, but I’ll ask.
I know it's not the answer you were looking for, but it seems like there may be a technical reason that Google doesn't keep track of the cached date for PDF files.