I don't think it's actually hanging, just taking a long time (at least that was the case in our tests), and then not retrieving much document text. So far, the only files we've seen that have this problem are our products' PDF files. As you found, removing .pdf from extensions to keyword avoids this delay.
Also, if you don't have a viewer configured for .pdf files [1], and 'Tools | Options | Editor | Show item text for documents with no viewer' is checked, the same PDF component is used to retrieve its text when an item is displayed in UR. This incurs the same delay and shows bogus text in the case of UltraRecall.pdf when it finishes. You can avoid this problem by unchecking that option or configuring another viewer for .pdf.
[1]
http://www.kinook.com/Forum/showthre...?threadid=2430