Jorgen,

This functionality is not in the built in Search Engine.

Jørgen-
If your content is "public" then an alternate search solution that handles searching of PDF and DOC content is the free Google Site search -- this is what I've switched to using on many of my clients' DNN sites.  Info on adding it to DNN along with a working example is here:
 
   http://googleAJAXsearch.com

The working example shows a "mutli-search" where you enter a term and get two sets of results:  one with matching web pages and a second of matching files.  The results appear "in-page" so you do not have to navigate away from your current web page to a separate "results" page.
  
The Google AJAX Search is capable of running alongside DNN search -- you do not have to remove DNN search though it might be confusing to have both. (PokerDIY.com currently uses both.)  You can test it out on a site just by adding the code inside of a Text/HTML module.

Note that this is not a good fit for all users.  For instance, your PDF and DOC files must be publically accessible (not require a login) in order for Google to index the file's content and include them in search results.  If you need to search "non-public" content on your DNN site then a DNNSearch-based solution is still the best route.
 
-mamlin