I've also been rethinking this logic if only because authors over on Archive of Our Own post a jillion of parts to a fic

Yep, to be honest I'm not sure how to save chapters into one complete file, so I do the same of d/l each individual chapter from Archive Of Our Own and uploading it to a folder in my kindle. It does get to be a pain though when some stories have like 20 individual chapters, so I should probably stop being lazy and figure out how to make a complete book with the fanfic I read :P