The current (3.9.9) reader front end does not appear to be entirely mobile responsive. It is fine for iPads but goes slightly off the page for iPhones. The Open Textbook plugin when activated works fine for both. Earlier versions of Pressbooks seemed to be OK.
Hi @MobiApp121, we are going to be redesigning the reader view in the next month or so this will certainly be addressed.
Good to Know thank you.