Google Drive app has recently added an Edit button when previewing a file. Xodo can be used to edit any PDF files. Here are the steps:
1. Select a file inside Google Drive App and click the Edit button.
2. Select Xodo from the choice of apps.
3. Xodo will open your document. You can now edit it.
4. After editing is done, click the system back button to return to Google Drive app.
Then click the system back button again to go back to the files list/grid.
Click on the edited document and you should see the changes you made in Xodo.
Alternatively, the System Picker can be used to edit Google Drive files as well: