PDFse.com offers a Google-powered search devoted exclusively to PDF files and ebooks. An extremely simple and stripped-down interface allows users to locate PDFs by search or by browsing a tag-cloud of previous searches.
A meander through items in the genealogy and genealogy-resources tag-cloud included varied things like an LOC-authored guide to local history and genealogy publishers, a USGS guide to using Geological Survey maps and resources in Genealogy, and familysearch.org's file on Jewish Genealogy Research.
If you're looking for PDFs with instructional or overview information, PDFse seems to be perfect. Various tests searching for actual PDFs of books didn't fair quite so well... books known to be present online did not show up despite using varied search parameters. Best to stick to archive.org and googlebooks for actual books, but for all else PDF-wise, give PDFse a try.