OS: Ubuntu 20.10
It is very annoying when the font of pdf changes to very bold, somehow I got it fixed from various solutions over internet but it popped up again. It now happens to some websites too not everytime but many times, but for pdf it's all the time.
You are not saying on which OS you are running. Could this be OS specific?
I can load a PDF if you point it out on the internet and see how it looks on my setup, but perhaps you have already done these kind of comparisons and know if this is related to just the OS/Setup you are running of if this is a behaviour that spans different OS's?
I wonder if this is a problem with Linux fonts.
I opened the PDF from your screenshot in Firefox Nightly 101 on macOS 11.6.4 and the fonts look fine.
I used Firefox's DevTools to see which font Firefox is using for the PDF: Tools menu > Browser Tools menu > Web Developer Tools menu item > Inspector pane. The "Fonts" pane says: "Times" and "Times Roman".