If it’s Adobe it can be made DRM-free relatively easy. It’s the first thing I do with every purchase (and legal where I live). Try Calibre (free/libre) or Epubor (paid but more versatile and works better for me).

Amazon has just introduced a policy change that probably makes it much harder (or impossible) to de-DRM stuff bought there, so I’m going to do without the books exclusive to them for the time being.

That’s why I’m driving a 1994 Geo Metro.

FWIW, Android offers a one-handed mode to shrink the available screen estate so you can reach the top of the screen with your thumb. It needs to be enabled in settings once and can then be toggled by double-tapping the home button or a swiping gesture at the bottom of the screen.
In my experience (6.1" Samsung Galaxy S2x user with slightly above-average hands) this is a good compromise between occasionally wanting to do things one-handed on the shrunken screen, and still being able to hit the right keys on the on-screen keyboard most of the time on the regular-size screen.

Bing DuckDuckGo says the iPhone has a similar feature, though I haven’t touched one in years so can’t say anything about it.