Cool tip that I relearned after forgetting how to do it.
If you need to search for a PDF, DOC or Powerpoint file from the Internet or a specific web domain you can use this syntax which will decrease the time it takes to get what you want enormously.

1. go to duckduckgo.com
2. type cats filetype:pdf
and then duckduckgo will search the web specifically for PDF's about Cats.

Damn this would be so cool if we could do this for other filetypes like AUDIO and VIDEO.

Or atleast we should be able to search for the Free Software equivalent formats like ODT and ODP.