FYI, I simply rotate the iPhone and read PDF’s (formatted as A4 paper) in landscape mode. Usually works ok. I also, when possible and that’s most of the time, create (via Print to DEVONthink) these PDF’s from Safari and/or Chrome while in Reader view.
Edit: I just installed the “free” version of Adobe Acrobat from the Apple Store on my iPhone. I discovered that their “Liquid” mode is available in the free version. And viewing DEVONthink documents in Acrobat appears to be quite simple using the Share button. So, best of both worlds I guess.