Unable to open downloaded PDF files directly from Opera browser

I'm using Xioami's MI 9 SE phone. I have several PDF viewer apps installed: Mi Doc Viewer, Pdf Viewer Plus, MuPDF viewer... However - whenever I try to open a PDF with one of these, or even with a file manipulation app, from a browser - I either get some error message (e.g. lack of permissions which I can't change, or some blurb in Chinese) - or just an empty screen. Here are two screenshots, of the errors with two apps, when I try to open a file via my (Opera) browser Downloads screen. The file I try to open is desktop.pdf; when pressing it, I'm presented with an app selection dialog, and whichever app I choose, it doesn't work. **With "MI Doc Viewer (Powered by WPS)"**: **With "Document Viewer"**: What might cause this problem, and how can I circumvent or fix it? PS - I don't get this for other file types (at least, not that I've noticed).