Enlarge Text

My sight is extremely poor so I have a Samsung Tab S so that I can get very close to the screen and read away from home.  I have just downloaded a book but I have not found how to enlarge the text and one page per screen is way too small for me.  The normal method of spreading my fingers does not work.

AND

I have also downloaded the book to my Ubuntu desktop which has a 23" monitor which I can pull close but the snag here is that only half of the page is visible and I cannot scroll down.

Thank you.