MuPDF mini doesn't have "Storage" permission by default
I don't have an app called "PdfViewer" on my system.
The default PDF app seems to be called "MuPDF mini" (com.artifex.mupdf.mini.app)
On my system, "MuPDF mini" doesn't have the "Storage" permission by default. And there is no system pop-up to inform the user that the "Storage" permission is required.
Thus, when I try to open a PDF from Firefox, the document is downloaded, and then the system displays the error "cannot open $FILENAME : permission denied". This is confusing because Firefox itself has the "Storage" permission, but that is irrelevant.