Font size won't fill the screen on Android

Please help.

The ebook from uni I have downloaded I have to read in tiny font, when I zoom it bounces back and will not fill the screen. Why will it not fill the screen?....

How can I read on a normal reader app that will allow the page to be displayed as the whole screen?.

Is it tied to just Adobe editions?

I tried to download into the phone but it won't open.

Adobe editions is not very good as far as I have used. 😐😐😐