In the Lumin app, open a document stored in My Documents.
When you upload files through Google Drive or Dropbox, they will sync automatically.
Open the document.
Once you have made edits to a file, simply tap on the menu button at the top-right corner.
Then, scroll down to Auto-Sync. Use this toggle to turn it on and off.
The app will then sync any annotations or edits made to the document with the copy stored in Google Drive.
NOTE: This feature only works on files stored under My Documents.