I just re-uploaded an “old” PDF, and the thumbnails are not working for an older file. The server that hosts our Omeka instance was updated about a month ago, so that must be the culprit. I’ll contact them to see what needs to be changed on the server. (By the way, I tried the “Test” button on the ImageMagick Directory Path in Settings, and it does say the ImageMagick directory path works…)