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.