Omeka Classic Thumbnails are Black sometimes

When generating an item from PDF files sometimes the thumbnails are created black or inverse. If I go through a long process of recreating the PDF using WORD then upload the file to generate an item the thumbnails will generate properly. Is there a fix for this issue?

I would first double-check that the path to ImageMagick (the open source tool Omeka uses for conversions like this) is configured correctly:

Beyond that, it’s possible that either ImageMagick or Omeka Classic itself is having compatibility issues with the PDFs that you are feeding it. It may also be worth asking on the Omeka Classic forums to see if anyone else has had issues like this: