Meh. Language is descriptive, not prescriptive

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.