have faced the same issue. pdf file is uploaded, but then one can’t download the file and goes to the “this page doesn’t exist” page!
I tested and I could easily upload-download pdf here on meta.
Update : I tried some changes and I guess this is more like a Contribute > Bug related to other languages rather than English.
I had to re-create the pdf with an English name (previously we had Persian characters in the file name). after that the upload can be downloaded without any problem.
Also, the pdf file with the Persian name could be easily uploaded and downloaded here on Meta . So I guess it has something to do with the Encoding option in the admin panel.