Maybe nine in ten of the files I open from the Reader bookshelf, PDF or text, don't display on an iPhone Xr running iOS 17.1.2. I see an empty screen like this:
On a newer iPad, PDF and text files open normally.
I'm seeing a bunch of weird behavior on an old phone with your very large flashcard database, actually, so I'm guessing this is part of that - will dig further and let you know if I need any other clarification, thanks!