The pre-publication paperbacks are marked 'Not for Resale' or some such, as noted above.
The fonts can be a headache, as the system is liable to silently substutute your fonts without giving any clear messaging. I belatedly realised something was going on when page makeup in "The Witch's Box" changed between the LibreOffice .doc paperback design files and the online proofing. With a little guesswork and experimenting I deduced that the font had changed to Times New Roman (same point size as before) and that my previous paperbacks were also in Times New Roman and not what was in the source files.
Actually Times New Roman looked fine to me, so I changed the font in the source to Times New Roman to keep the page makeup consistent.
I recommend culling errors digitally by spellchecking and using ProwritingAid on the source text, then keep a look out for 'wrong word' typos that evade spell check, and at digital proof stage look out for formatting errors, e.g. a short line justifying to full width. With the paperback proof you can check the 'real book' look, whether the cover aligns as you expected, and have a final check for anything that may have got through the previous checks.