Comment 3 for bug 1427694

FYI, calibre inscludes an ebook editor that can edit epub files
directly, which you should use rather than zip/unzip as the epub file
has special internal conventions that zip/unzip will not respect.

Also note that splitting does happen on many different types
of "suitable" locations in the HTML, not just page breaks. However, it
is not always possible to find suitable locations automatically.