I searched and the last thing I found on this topic Thumbnails for PDF files are not being created said that a more secure solution was being sought. All our existing pdfs are displaying just fine with proper thumbnail images, but I loaded one up this morning and I get nothing. https://consortiumlibrary.org/archives/omeka/exhibits/show/ascrestaurantmenus/anchorage-restaurants I’m not the tech guru who runs our servers but I can request that any server issues be done by that person, if I can get pretty exact instructions.
Thumbnails working for everything but PDFs usually indicates that Ghostscript is not installed or that ImageMagick’s PDF support has been explicitly disabled.
Someone with server access can try using the
identify commands on a PDF to see if ImageMagick works on PDFs.