Yeah, I think back button should be tied to the article. As an experiment, I flipped through a half dozen pages, then back a couple, then forward another half dozen, then back a few, then forward a few, etc. like I was doing research and correlating information from different part of the article...escaping the article was pretty exhausting after all that ;)

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.