Quick Guide for Android

1. When you want to open the file, tap the More icon or long tap the file;

2. Choose Open in Other App; 

3. Tick PDF Reader Pro and choose Always;

Or if you have set other app as the default PDF viewer, please do as follows first, then try the steps above:

1. Open the Settings app;

2. Tap Apps & Notifications;

3. Tap the app that you set as the default PDF viewer, tap Open by Default and then choose Clear Defaults.