Documents offers most of its functionality for free. However, there are superpowers in the Documents Plus subscription, for you to do even more with your files:
Important note for existing subscribers: Those of you who previously subscribed to professional PDF tools in Documents can still benefit from these features on the terms and price you had before. You can also upgrade your plan to get a VPN.
Unless you have already upgraded, you'll be offered to start with the Documents Plus subscription every time you try using the VPN or advanced PDF features (i.e. the functionality that comes as a part of the subscription). You'll be upgraded once you review the subscription terms and confirm the action in the Apple's pop-up message.
The subscription is only required for the professional PDF features and VPN. You do not need a subscription to manage your files, watch movies, read books, listen to music, download files, connect to a PC, transfer files, etc. All these features are available to you for free, without any limitations.