What is the difference between free and paid account?

There is only one difference between a free account and a paid one.

A free account lets you sign documents that are sent to you.

A paid account allows you to send out document folios for signing to other people in addition to that. When you are subscribed to a paid plan, you have a limited number of documents folios that you can send out per month. If you reached the limit, you can increase it by upgrading your plan or purchasing one-time sends on a pay-as-you-go basis.