Great question! With a premium account you can create a SideDrawer for your dad and make him the owner and, with his permission, still get access through your own SideDrawer account.
Keep in mind that, as the owner, your dad will have the ability to remove your permissions to his SideDrawer.
You can choose to pay for your dad’s SideDrawer or simply ask him to pay for it himself. Now you can fully manage his documents!