Try opening it in the browser. Go to the address bar and put in file:///C:/path/to/file (if you’re on Windows, which I’m assuming you are). Firefox should be smart enough to recognize it as a pdf (if it indeed is) and open it, with or without extension. If it won’t open it, you’ll probably get an error message, which could shed more light on the problem.

Soul mates aren’t found, but made.

But the F in FOSS stands for free. I understand that there’s a lot more to unpack in the OS part of FOSS, but still, it’s not quite wrong.