Hi there,
Lately, there seems to be a swath of people (outside of Cloudron, at least) reporting their PDFs no longer get thumbnails generated. I recently had to force regenerate all thumbnails on one of my WP sites on Cloudron, and faced the same issue: PDFs no longer get thumbnails.
While investigating, it seems that ImageMagick could be the culprit here. I think WordPress uses ImageMagick to do the conversion. Using wp media regenerate outputs lines like:
Warning: attempt to perform an operation not allowed by the security policy `PDF' @ error/constitute.c/IsCoderAuthorized/408 (ID 5110)
Warning: No metadata. (ID 5110)
3/558 Couldn't regenerate thumbnails for "<redacted>" (ID 5110).
I traced it to the /etc/ImageMagick-6/policy.xml file that must've changed recently in the base image used by Cloudron. Could you perhaps look into it? I'm using PDF thumbnails heavily on one of my hosted sites and it used to work just fine 
Thanks in advance!
All the best,
Vince.