Ref: Mpdf plugin - unmaintained?
Current version of Pressbooks now breaks with this plugin when generating a PDF. Issues now appear such as ignoring PDF custom styles and short header title.
Alas, sad that BCCampus has not maintained this excellent plugin, as it provided Pressbooks stand-alone ability and is part of the FOSS movement; whereas Prince is a commercial product. Such a loss puts into question the risks inherent in adopting open source plugins that depend on a particular version of Pressbooks to function correctly.