1. Search the internet for a pdf image, preferably a jpg or gif of size 16X16. You can use the image below as well.
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2. Copy the pdf image into the images in the below location
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\IMAGES
3. Find in the DOCICON.xml file present in the below location
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\XML
4. Add the values <mapping key="pdf" value="yourimagefilename"> in the <byextension></byextension> section.
5. Reset IIS.
6. Search for any pdf file on the portal and the image should be visible now.