Here's a potentially useful search engine if you're specifically looking for documents:?http://www.findthatfile.com
findthatfile.com?allows you to narrow your search by filetype in a way that is a little bit more powerful than Google's own advanced search facility (and more intuitive). Filetypes include?PDFs, documents (DOC, TXT, etc), audio, video, RAR and ZIP compressed files.?
(Strangely spreadsheets are not included, for which you might want to try the excellent Zanran).
The site also has an API, which may be useful if you want to find documents related to a long list kept in a spreadsheet.
Nicole Boivin from Find That File?says:
"We open each file, identify its author, title, contents, text extracts and all kinds of goodies that nobody else does.? We also search more places than anyone else : Web, FTP, Usenet, Metalink and P2P (ed2k/emule) including 47 file types and 557+ file extensions including over 239 file upload services."