In Apple Book iBook, you can't change the text size of pdf files; If it is an epub file, you can change it by clicking the "AA" icon in the upper right corner after the iBook is opened.

Books is a tool for reading and buying books on various Apple devices. Download the free iBooks application from the App Store, and then users can get all the classics and bestsellers from the built-in iBookstore.

System requirements: compatible with some models of iPhone, iPod touch, iPad and Mac. IOS devices need iOS 5.0 or higher, and Mac devices need Mac OS X 10.9 or higher.