I can’t get any thumbnails to show up for newly imported (or indexed) PDF documents. I get plain white thumbnails. I can delete the thumbnails (and then I get the generic PDF icon) and then create the thumbnails, which again results in a plan white thumbnail.
On old files, I have thumbnails, but if do delete/create or update they turn into plain white.
This is unfortunately an incompatibility of the new PDF thumbnailing of version 2.9.12 which renders also annotations. The upcoming version 2.9.13 will fix this, in the meantime the only workaround is to downgrade to version 2.9.11.