On September 16, ten days ago, I got a relapse and started to contribute to Wikisource again. I found some nice PDF files and uploaded them to Commons, a total of 81 issues of the official gazette of Finland from the winter 1878. I have since then been proofreading several pages from the first week of March (when the peace at San Stefano was signed), e.g. https://sv.wikisource.org/wiki/Finlands_Allm%C3%A4nna_Tidning_1878-03-05
But previews (thumbnails) have been generated for none of these 81 PDF files. I am able to proofread the text (embedded in the PDF) only because I keep local copies of the PDFs that I can view. https://commons.wikimedia.org/wiki/Category:Finlands_Allm%C3%A4nna_Tidning_1...
On September 17, I asked about this problem on the Help desk, https://commons.wikimedia.org/wiki/Commons:Help_desk#PDF_previews_missing and a kind person registered the problem in Phabricator, https://phabricator.wikimedia.org/T151202 but then nothing has happened in more than a week. So there is not only a problem in generating thumbnails, but also in responding to a reported problem.
I don't even know if there is a technical problem with the PDF files I uploaded or if this is a temporary technical problem on Commons. (But since I can view the local files, I assume the files are okay.)