One problem normaly people ask, is "Why PDF, dont have a ico associated to file", well simply answer, thats because is not a microsoft file...
Well to resolve this question have this litle tutorial to add the pdf image to document library file.
Copy the ICPDF.GIF () file to
"C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\Template\Images"
Edit the file
C:\Program Files\Common Files\Microsoft Shared\Web server extensions\12\Template\Xml\DOCICON.XML
Add an entry for the .pdf extension
<Mapping Key="pdf" Value="icpdf.gif" />