Web search engines can be used as tools to search for literature not published within the main channels of literature indexing or supply. The best value in web searching is to search within known sites or to use a search engine's advanced search features.
Google has a number of advanced search features which can make a search much more targeted. These features include searching by:
File type searching:
All of these operators can be mixed together as a search. For example - food security site:fao.org filetype:pdf