I’ve done it both ways. I’ve used PDF’s stored in Glide storage as well as PDF’s stored externally. As long as the PDF url is publicly accessible. If you look at the following post, I did it both ways in that test. When wrapping the PDF url, it’s using Google’s viewer in the web embed component, but Google’s viewer can render PDF’s stored anywhere publicly.
I will say this though. It can be a bit finicky at times and not load the PDF every time, but going out and back in usually works.