For those of you who haven't heard about it yet, the Portable Document Format (pdf) is now officially an ISO standard. Read more about it on Betsy Fannings blog post.
There are heaps of resources out there regarding installing and setting up of pdf ifilter. But since you're here, I'll walk you through it as well.
Firstly, do a Google search for "Adobe Pdf ifilter" and then download the latest/correct version of it. Now, prior to installing the ifilter, stop the IIS Admin Service from the Services console.

Now, install the ifilter by following the on-screen prompts. Then navigate to the TEMPLATE\IMAGES folder under "\Program Files\Common Files\Microsoft Shared\web server extensions\12" aka the 12 Hive. Copy in your pdf icon image here. Download and use this icon if you wish (pdficon.gif).

Then still within the 12 Hive navigate to TEMPLATE\XML folder and open the DOCICON.XML file for editing. Paste in this line into the file.
<Mapping Key="pdf" Value="pdficon.gif"/>
Save and close the file, then Restart your IIS Admin Service.
Go into your Shared Services Provider SSP, under Search Settings --> Crawl Settings click on the File types link. Click New File Type and add "pdf" then click ok. Perform a crawl if required. Try searching for a pdf document and it should pick it up.