Drawboard PDF's single page view allows one to swipe through pages (like "flipping" pages in a book). I find this helps with retention as it breaks apart the text, delineated by page breaks. I've tried your single page vertical scrolling view, but I found it to be clunky. A few things that would probably make this better (for the horizontal swipe as well):

1. Don't have the page "glued" to the screen, but put it on a rubber band -- so, as someone "drags" the page, you can see it react/ shift from the center/ default position. If the user just drags, then releases, the page will snap back into place. If the user swipes (ie, gives the page some momentum with a finger flick) with a certain amount of force/velocity, then the page "flips" to the next/previous one.

2. One feature I like about Xodo PDF (on Android)'s experience with the single page/ horizontal flipping of pages -- if you are zoomed in to the page to read the text (as you frequently are on a smaller phone screen), if you flick forward, it automatically places you at the top of the next page, at the same zoom level, at the same horizontal scrolling position (or, maybe it just centers on the text)? Likewise, if you flick backward, it shows you the previous page, at the very bottom of the page. I like this because you can keep reading without having to adjust your page position any more. This is probably a little less of an issue for the larger screen sizes of desktop/ laptop computers, but would still be nice to have in case one is zoomed in some and wants to keep reading at that zoom level from page to page.

As for why I'd prefer a horizontal swipe vs a vertical one -- this is more intuitive for me, having been trained on books when I read very long documents. Again, having this be horizontal helps me with retention (I feel)