If you have Android 4.4+ device, you will open documents as read-only mode when you open them from external SD card.
Because accessing to external SD card is limited to read-only from Android 4.4 by Google.
So, how to open documents as writable mode in the external SD card ?
1. Select Google Drive in top screen
2. Select "Settings" item in the action overflow
3. Check "Show advanced device opt." box
4. You can get writable external SD card
You can edit documents if you open form here.