Brief investigation suggests reflow is a super-clumsy, ultra-coarse-grained view mode that is implemented by few clients, is not easy to access, is not well known, and is vastly inferior to what you can get on the web, especially as it’s basically text-only.

In Adobe Acrobat (and I’m guessing Adobe Reader): Choose View → Zoom → Reflow, and it turns everything into one column of nigh-unformatted text.

(Word looks like it may support it, but that could be more that it’s converted it to a Word document in some way and reflow-like functionality falls out of that naturally, though I imagine the tagging would help with the conversion; and someone in this thread mentions something called “Book Reader” supporting it.)