Today I got so annyed by firefox's default behaviour of downloading each and every PDF file to my disk that I went searching for a solution until I had the problem fixed. And it seems like I have finally found it. I have linked the solution but here is the fix in short:
go to about:config
change browser.download.open_pdf_attachments_inline to true
After watching other comments I wonder why some people don't get that this was a bug or at the very least, undocumented behavior outside of the Settings GUI. I had to deal many times with file (4).pdf on my Downloads dir. A BIG thank you, OP.