I had a similar issue a few days back, came out to be that PDF thumbnails
were created from a directory listing, which meant that CF would try to get
a thumbnail out of every file in that directory. Since you mentioned
no error, not sure if thats your case, but try making sure there is no
other file in the directory the PDF files for which the thumbnails will be
created are. Sometimes MAC os, creates hidden property files for each
folder, try listing the directory and dumping the result just to make sure.