I have been reading some posts about auto-generating thumbnails for PDF files. It seems as though the issue generally relates to Imagemagick or Ghostscript.
I was wondering, however, if embedded PDFs (linked from another site) will auto-generate a thumbnail. I have not had any luck thus far.
I have the same problem. Pdf added as item does generates this kind of thumbnail (image below). I sort of expect to see the same thumbnail with pdf in Omeka Classic.