In T223357#5186450, @Aklapper wrote:

That makes sense but is another topic.
Feel free to file a separate "When a thumbnail is not created due to PDF file corruption, display an error message instead of silently failing" task or such.

I manually added that message about the file being "broken or corrupt" when I noticed the issue, but there is nothing wrong with the file when viewing it traditionally.
Ideally, PdfHandler would be able to gracefully fail and at least show some information in the thumbnail rather than simply breaking. Why is PdfHandler less capable than any browser's built-in PDF viewer? See how it loads fine at https://upload.wikimedia.org/wikipedia/commons/4/47/Mueller_letter_to_Barr_2019-03-27.pdf?