Setting up your SharePoint farm to be able to crawl PDF files and show the PDF icon is a manual task which involves changing the registry.

If you don’t want to go through the steps manually, try the stsadm command from here: http://notesfor.net/post/2009/07/14/Automate-iFilter-and-PDF-Indexing-support-to-a-SharePoint-Farm.aspx

You can find the CodePlex site here: http://notesforsearchfilter.codeplex.com/

After the stsadm extension has been installed, setting up PDF support is easy:

   1: stsadm -o searchfilter -ext pdf -filter PDFlib –iconfile "C:\Temp\icpdf.gif"