Steps to replicate? Can we access the same file? X11 or Wayland? What desktop? Do you have the same issue with Firefox in Troubleshoot Mode?

Try downloading Firefox from Mozilla. Download, unzip, and run firefox-bin from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Now I noticed it happens from time to time. And I don't know exact steps to reproduce.

Hmm, do you use any custom CSS (userContent.css or userChrome.css file)?

Yes I have DarkReader, but it doesn't work in PDF I think.

Add-ons aren't allowed to inject style changes into the built-in PDF viewer, although they can modify the overlaid custom viewers displayed on some websites (like Google Drive). So changes would need to be made by a userContent.css or userChrome.css file -- and if those don't sound familiar, you probably haven't installed one.