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 FilesCommon FilesMicrosoft SharedWeb server extensions14TemplateImages
- Update the Docicon.xml file in the following folder C:Program FilesCommon FilesMicrosoft SharedWeb server extensions14TemplateXml to include the following node:
- Restart IIS after making the change
Details on how to perform this for all versions of SharePoint are available at this Microsoft KnowledgeBase article.