Also back button behavior is frustrating and when paging backwards the pages slide in in an unexpected way that breaks the book metaphor
"Back Button" is also a bit tricky. ^^ As the other commenter said, it may be better to record URLS per articles instead of page unit. Will that be more natural? Thank you again.
For progress, you might want to consider checking out the internet archive e-book reader as a template. https://www.archive.org/stream/birdbookillustra00reedrich#pa...
The progress bar on the bottom is amazing. Chapters (sections) in a text are noted with, you can click anywhere on the progress bar to "fast-forward" and as you turn pages, it progresses, giving you a sense of where you are in the text (which would help with your page-up/down behavior).
They also offer several ways of turning pages, click (to turn left, right), left-right GUI buttons and left-right on the keyboard (page-up/down turns only one page).
More importantly, they offer 1-up with a vertical scroll, which I'm finding as a metaphor I love on mobile, which lets you drag to scroll (and the left-right GUI buttons and progress bar all still work), though they change the keyboard controls to up-down to scroll.
I wish I could find the example, but I can also tell you that their reader handles different widths of pages. So in 1-up mode, most pages can be book-paged size, but if there's an extra-wide page (like a poster or a fold-out) it will expand to accommodate that...which could be a useful metaphor for handling wide tables.
Just some ideas.
edit
I noticed mobile also changes pages on tap, but the behavior wasn't consistent. I had to tap 6 or 7 times to get it to work if it worked at all.