Functions of both versions are equal, but in free version you can try all features on 5 documents and then have to activate subscription. Free version provides several payment options:

1. 1 month auto renewable subscription
2. 12 months  auto renewable subscription
3. Lifetime in-app, when you pay once and get unlimited access to all features and updates. Lifetime purchase of free app is equal to buying a pro-version.

 Features of pro-version are equal to free version with activated subscription.