The reason why the pdf imported by Ios book app is not displayed.
1. Open the APPstore of Apple's mobile phone and download the brisk pdf reader to Apple's mobile phone. Open the software and click "+"in the upper right corner to add a pdf file. Apple's mobile phone supports iCloud (cloud) direct import, and there is no need to import pdf files into the mobile phone to open it.
2. Click on the pdf file to be edited for reading.