The best and biggest search engine.You can search for PDF files by entetering "filetype:pdf" after your keywords.Eg:- nanotube filetype:pdf for word file nanotube filetype:doc You can use this by from Advanced Search option and select the filetype that you needed.Google supports,Powerpoint,pdf,word (doc),rich text,excel etc.You can restrict your search by choosing Language from advanced search.
The search engine that helps you find relevant information, video, images, and answers from all across the web.You can use the advanced search option to search for pdf/doc files only by selecting the format that you needed. Eg:-nanotube pdf files only.
Bing (formerly MSN Search and Live Search)
Bing is a search engine that finds and organizes the answers you need so you can make faster, more informed decisions.
Offers search for web sites, images, news, blogs, video, maps and directions, local search and shopping.
Question and answer search
Now the popularity of yahoo answers shows people need help on their area of research.So here some human answers and automatic answers search engines are shown,these will really helps you.
Human answers search engines
The ultimate answer engine, with quick accurate dictionary, thesaurus, encyclopedia, bios, tech terms, news, sports, weather, and much more.
Learn how to do just about everything at eHow, the world's most popular place to find How To instructions.
Ask questions and browse existing answers from experts in various categories.
You can ask questions on any topic, get answers from real people, and share your insights.
A language-independent collaboratively edited question and answer site for programmers.
DeeperWeb is an innovative search engine plugin and an essential Firefox addon for Google.