Cool feature, although I note with some irony that once I have the PDF downloaded, I still have to then use Google's Drive PDF reader or some other tool to open it. Maybe work on a full stack PDF renderer like the desktop version has, Mozilla?
This is in the works. It's the same codebase as the desktop version, which is written in js. I've tested it on low end Android phones from 2017, it works well, when compared to the native offering, and also on modern high end phones, it also works well of course. What's missing is mobile specific UX, such as pinch-to-zoom, and UX/UI refinements, so it's nice to use. It has form support, annotations and other goodies…
Did you have to enable anything else?