Pdfgeni helps you zero in on the PDF you're looking for. It's a dedicated PDF search engine; just enter a search string, and you'll get pages of results, all in PDF format.
I've found some truly useful things with Pdfgeni, like a compact version of Ken Rockwell's user guide for the Nikon D200 and a white paper from IDC on the value of video collaboration (one of my personal pet projects).
Pdfgeni is not entirely unlike The Manuals, though Pdfgeni is more open-ended, and will find any PDF it can; The Manuals is designed to sniff out user guides exclusively.
Add Pdfgeni to the list of oddball search engines you use instead of Google or Bing for specialized searches -- you, know, like Wolfram Alpha, or this collection of ten specialized search engines that look for free stuff, documents, video, and more.