Enlarging text depends on the book format and device type.
On Android and iOS devices:
- ePUB formatted ebooks: Once you open the book, navigate to the Settings panel in the upper right corner, and in that menu you will see a Text Size control. tap on the "+" button until the desired text size is attained. Tip: on an iPhone you can rotate your device to see more text under/next to the settings panel to see your text size.
- PDF formatted ebooks: Since PDF is a fixed layout format the best way to enlarge the text is to use the pinch zoom technique as there are no settings that would allow for actual change the text size of a PDF formatted e-book.
For Windows devices, you can open the book and select View > Zoom to see your options.