I did manage to open the html files using some android html ide (sounds dreadful tbh), because you basically need to run a server as far as I understand. Other methods I haven’t tried, but that probably work just as well are running termux and then hosting the files in a temporary server, for example through python or whatever, and the one that could potentially work the best out of these is just hosting a regular, private server on a rented or otherwise machine and then connecting to that with your android device.
However, the default experience isn’t great. It’s quite hard to control and selecting text is annoying at best. So probably the best would be doing the server method from earilier (basically plex but for books) and then coding up a better android ready viewer for it.
As for the pdf, I just tried stuff like adobe acrobat reader, google drive reader etc etc, also the same annoying text selection issues. And you either don’t see the text and therefore you won’t know if it’s incorrectly parsed until you copy it out, or you have it overlayed, which isn’t the best. But it did work with those I think.