Not sure if this is an Omeka issue or a PDF one but would greatly appreciate any advice - recently installed Omeka after a long search for something to solve my particular archiving project. Funny thing was that Omeka was one of the first products I looked at but I had to work though a truck load of other products to work out that it was the right one all along. Anyways I have one particular file I have uploaded that simply refuses to produce a thumbnail of the cover. I have reprocessed the PDF several times and tried the usual suspects of removing hidden objects and flattening everything (what does get rendered in the thumbnail does actually look like some hidden objects but I removed them) but no joy. I know the process works because all previous documents are just fine. Another thread on this was not of much help sorry Blank (white) thumbnails Here 'tis - http://trs-80.computer/items/show/109 - thank you!!!
The issue here is probably that the image embedded in that PDF is in JPEG2000 format, and your ImageMagick/Ghostscript install doesn’t support JPEG2000 images.
If you control the server it’s likely just a case of updating or reinstalling Ghostscript. If you have a host, ask them about it.