For mobile devices such as smartphones and tablets the controls for the bookreader are found by clicking the "hamburger" (three horizontal lines) icon in the upper right side.
To enlarge the text follow these steps:
1. Use the fullscreen mode by clicking the icon in the lower navigation bar
2. Click the "hamburger icon in the upper left
3. Click the "Settings" link
4. Click the zoom magnifier icon to enlarge the text