Deploy SharePoint and chances are you will get asked to set up icons for PDF files every time. Do it twice and you build a batch file, add it to your "Deployment Kit" and call it a day.
I had an issue at a client site where on one development server there were no icons showing up in the Search results. Not for Word, Excel or PDF. Nothing. It turned out that a second (duplicate) entry had been made in the DocIcon.xml file for PDF’s – so while this was not affecting the icons associated with files in Document Libraries, the search results screen didn’t like the duplicate information – and so stopped showing icons for all results (in the HTML, it showed an image tag with an empty src attribute).
Removing the duplicate tag solved the problem, but it took a little while to work out what it was so hopefully this will be useful to someone. There’s a bit more about Search Results PDF Icons not appearing in this KB Article as well (although it does not mention this specific problem) – http://support.microsoft.com/kb/832809/en-us?spid=11373&sid=global