When you connect the Lumin PDF app to Google Drive, you can select Lumin PDF as your default viewer for easy viewing, and, annotation of PDF documents.
Enabling as Default:
1. First, ensure you have installed the Lumin PDF app to your Google Drive. Find out how here
2. Go to your Google Drive: drive.google.com
3. Click 'Manage Apps'
4. Find Lumin PDF
5. Next to the Lumin PDF app, to the left of "Use by default," check the box.
Disabling as Default:
To disable Lumin as the default app follow the above steps, and ensure that in Step 5 the box is unticked