I was just surfing the net and came across a site which is limited its search to specific files such as Word document, PPT, Excel and PDF file.
The site is specifically a file search engine.
It is a good tool and the results generated are useful. I searched for "Download The monk who sold his Ferrari" in PDF Section. And this is what the result was.
Link to the site - Brupt.com
This is a good start up I say as the search is defined. I didn't see for any other such site (site on file search) as I liked this one very much.
Investment I cost of this site I assume is minimum and the revenue generated is mostly/mainly from Google Adsense for Search (Ads). They are looking for various other sponsors.
This is yet a growing site. Can gain popularity very soon.