When I have time, I format books to convert to LRF--use Heading styles for the chapters, convert footnotes to something that shows up properly, and so on. I either need enough time to deal with BookDesigner, which means careful runthrough of the whole book, or time to format & convert to HTML, which I'm still not comfortable with, to throw it into Calibre (otherwise, I keep my bold/italic/etc, but lose chapters & page breaks; I still haven't figured out if there's any way to go from RTF to chaptered, TOC'd book with Calibre).
When I want a quick conversion, I use a Word doc with roughly three-and-a-half by four-and-three-quarters inch pages (can't remember the exact numbers), .15" margins all around, put the whole thing in 10 pt
Fontin condensed by .1 pt, put page breaks before the chapter headers (which I haven't decided whether they should be 12 pt or 14 pt) in bold & centered, and play with a few more settings like footnotes and prologue and so on.
Convert to PDF. If I took the time to make the Chapter labels "Headings," they'll be bookmarked; if not, I do a quick run-through to add bookmarks. Change metadata to what I like. Then load it onto my (currently very tiny) card and it's good to go. Don't need reflow 'cos I've picked the size that I like to read.
I'm aware that, if I practiced more with BD or HTML programs, it'd be as fast or faster to work with them. I've been working with PDFs for over eight years; I'm very comfortable & quick setting them up.