By default SharePoint uses a white icon for PDF documents. It’s rather plain. To use a PDF icon follow these steps…
Download the PDF icon to your SharePoint server.
Navigate to C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\IMAGES
Place the image in the IMAGES folder and name it icpdf.gif
Edit C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\XML\DocIcon.xml
Insert <Mapping Key=”pdf” Value=”icpdf.gif”/> between the following lines…
<Mapping Key=”onetoc2″ Value=”icont.png” EditText=”Microsoft OneNote” OpenControl=”SharePoint.OpenDocuments”/>
<Mapping Key=”pdf” Value=”icpdf.gif”/>
<Mapping Key=”png” Value=”icpng.gif”/>
Save the file and restart IIS.
Update: Make sure the quotes are correct when you paste the text into the configuration file. Compare them with the rest of the quotes in the document.
Source: http://www.sharepointedutech.com/2010/02/17/changing-file-associations-icons-in-sharepoint-2010/