1> Stop the IIS.
2> Install the Adobe IFilter from the below links -
http://www.adobe.com/support/downloads/detail.jsp?ftpID=2611 for 32 bit
http://www.adobe.com/support/downloads/detail.jsp?ftpID=4025 for 64 bit
3> Then copy/add the icon of the pdf to "c:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\IMAGES"
4> Then go to "C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\XML" and edit the DOCICON.XML
5> Add the key that is mentioned below inside the above XML file.
6> Restart the IIS.
7> Then go to Central Administration page to add the file type. Go to the SSP, then click on File types list.
8> Click on new file type. Type in PDF, then click OK.
Finally, you get to see the pdf icons for the PDF files that you have uploaded on the sharepoint site.