1- Find a PDF icon. Let's say 'PDF16.GIF'. Copy the file 'PDF16.GIF' to 'C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\Template\images' folder.

2- Open the file 'C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\Template\XML\DocIcon.xml' and add this line : '< Mapping Key="pdf" Value="pdf16.gif"/ >'

3- Perform an IISRESET

This will display the pdf icon along with pdf files. As Sympmarc said the pdf Ifilter will help you index the content of pdf files so it can be searched.

Hope this helps.