The file https://commons.wikimedia.org/wiki/File:Montazem_Naseri.pdf was uploaded with high compression, size 25.5 MB. The file did not display (jpg preview, thumbnail, next page option -- nowhere). Then I decompressed it to 178.68 MB and overwrote the previous file. Now it displays. The thumbnail of the previous file is still not visible.
The first version uploaded wasn't displaying thumbnail pages—choosing whichever you wanted, tried and confirmed at random pages—and this was either directly in the File: at Commons, or when viewed through some of the variations available at the Wikisources.
I have a similar problem with this file: https://commons.wikimedia.org/wiki/File:An_Anglo-Chinese_Vocabulary_of_the_Ningpo_Dialect.pdf
The thumbnail is not displayed even though the file is readable after being downloaded.
I'm pretty sure that the file is a valid PDF because it works on my laptop. It also previewed well at the source site: https://archive.org/details/wilhelmgesenius00gese .