The sharepoint Search Service uses the indexing capabilities like the OS Indexing service does. This means that you can extend it’s capability by implementing the IFilter interface. This is an interface that the indexing service uses to get text from files and so on. So you can implement it to support any file type you like as long as you yourself know how to parse it to text :-). Lukily for me Adobe has been so kind to provide an implemented version of the IFilter for pdf documents.
So you go to http://www.adobe.com/support/downloads/detail.jsp?ftpID=2611
There you can download the Ifilter and install it.
After installing you’ve enabled the indexing service to use it, but you still need to make sure that sharepoint can use it. To do this you will have to do some registry hacks:
1. Open regedit and go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\12.0\Search\Applications\>\Gather\Search\
Extensions\ExtensionList (mind the line break!)
Note: The is different on different systems anyways it should like {XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}
2. Create a new String Value (Edit –> New –> String Value)
3. Rename the new string value to 38
4. Right click 38 and choose Modify
5. Set it’s value to pdf and click OK.
Ok you’re set. Now upload some pdf documents to your wss site and after that restart the Sharepoint Search Service using net stop spsearch to stop the service and net start spsearch to start the service. (oh yeah you run these commands in a command prompt ofcourse 
start -> run -> type cmd and hit enter)
Et voila pdf indexing and search has been enabled on your wss setup. You should give the system some time to index the files.
Note:If there were already pdf’s uploaded to the wss site before you installed the IFilter there is still a small problem. These files won’t get indexed automaticaly, but you can trigger indexing by changing just any property of the file. For example change the title of the document. Another way is to just upload the files again.
Ok before you think that I’m some sort of Guru on searching, indexing or even sharepoint please note that all that I mentioned here can be found in the following KB article http://support.microsoft.com/kb/927675
