Why was a PDF website automatically download onto Firefox as a file?

I clicked on a PDF website while searching for articles on relations between Ukrainian settlers and First Nations people. Instead of opening so I could read it, it was automatically downloaded as a file into Firefox. Unfortunately, I noticed the title had random parentheses in AFTER I clicked on it to read it. I updated Windows defender, and ran the offline scan. I also uploaded and ran MSN.com M.S.R.T. and Malware Bytes, but they found nothing. What happened? Did I unwittingly install Russian malware into my computer, or does Fire Fox automatically download some PDF websites as files? Thanks, Charlotte