1. Open DOCICON.XML file under C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\XML\ folder.
2. Add entry to the file as mentioned below (on all WFE's and Index servers)
<Mapping Key="pdf" Value="icpdf.gif" EditText="Adobe Acrobat" OpenControl="SharePoint.OpenDocuments" />
Note: EditText setting above will not work with Adobe Reader, it need to have professional version.
3. Get a 16x16 size pdf icon file from adobe (should be available on their site of just google it) and paste it to the below folder:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\IMAGES\
Make sure the name of the icon is 'icpdf.gif' or if you have it as xyz.gif then change the Value in above mapping tag.
Thats it! Simple isn't it?