Paypal's REST API documentation (https://developer.paypal.com/docs/rest/api/) is really nice. Is this generated with a tool, or manually? If there is a tool used, what is it?
Paypal is using iodocs for API explorer. Here's the tweet from Paypal developer about the same. This is originally forked from an open source project iodocs by mashery.
Checkout more API explorers here similar to iodocs.