If you're unable to adjust the font size in the OverDrive app on an iPad (on iOS 10), this may be caused by an issue with iOS 10. To resolve the issue:
- Update your device to iOS 10.2 (or higher).
- Update the OverDrive app to 3.6.1 (or higher).
- Open the eBook in the OverDrive app and try adjusting the font.