How to add an PDF icon to MOSS document libraries

To specify the .gif file that you want to use for the icon that represents PDF documents in yours Document librairies, follows theses steps:

1. Copy the .gif file that you want to use for the icon to the following folder on the server:
Drive:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\60\Template\Images

2. Edit the Docicon.xml file (located on \Program Files\Common Files\Microsoft Shared\Web server extensions\60\Template\Xml or \Program Files\Common Files\Microsoft Shared\Web server extensions\12\Template\Xml ) and add the following line in the < ByExtension > section

< key="pdf" value="MyPicture.gif">
*replace "MyPicture.gif" by the name of your picture

3. IISRESET

Reference : http://support.microsoft.com/kb/837849