When a document library includes PDF files, by default these file’s icons are displayed with the generic “blank” icon.
- Upload the .gif file for the PDF icon to C:\Program Files\Common Files\Microsoft Shared\Web server extensions\14\Template\Images
- Update the Docicon.xml file in the following folder C:\Program Files\Common Files\Microsoft Shared\Web server extensions\14\Template\Xml\ to include the following node: <Mapping Key="pdf" Value="NameofIconFile.gif"/>
- Restart IIS after making the change
Details on how to perform this for all versions of SharePoint are available at this Microsoft KnowledgeBase article.