This is the official method. It is safe and recognized by Steam. Cons: You still need the Steam client to "Restore" these files later, and it is primarily meant for backup, not necessarily for playing strictly offline immediately.
If you want a truly portable version that doesn't require a lengthy restoration process, you can simply copy the game folder. Dota 2 Offline Installer
You couldn’t patch. You couldn’t queue. The “Reconnect” button was a cruel, gray liar. This is the official method