I dunno since when, but upload of PDF images fails, this hurts especially on tex.SE. Could this be an effect of an updated
/etc/ImageMagick-6/policy.xml on the imgur server? At least on my Ubuntu 18.04 LTS machine, this file was modified two weeks ago.
There is a Q&A on Tex.SE's Meta: "Add directly a PDF image from overleaf to a question or an answer edit" about uploading .PDFs:
When posting a question or an answer on tex.stackexchange.com I often start by creating a MWE with overleaf. Then if I want to include the resulting PDF I have to download it to my local system first before inserting it into the post. My question is as follows: is there a way to include the PDF directly from overleaf without download it to my local system?
As of now, Stack Exchange only provides image uploader on all sites using Imgur's service.
While some SE sites got their customization like Code Snippet (SO, etc), MathJax (Mathematics.SE, etc), embedded YouTube video (SciFi.SE, etc), ABC notation (Music.SE)... but PDF uploader is not one of them.
BTW, I do see an error message when I attempted to upload a .PDF to Tex.SE using an URL, attempting to select a file via the browser fails silently.
There is a bit of difficulty uploading .TIFF files also, the preview doesn't display, though the file uploads and display's correctly. The current dialog says that "jpeg, png, tiff, gif or bmp" are accepted but it also accepts .WEBP files for upload and correctly shows the preview; it's only when you hit the Add Picture button that it fails.