Anybody has experience integrate Netsuite with e-sign app?

Integrated adobe sign many years ago

Do you know how to send the document to that e-sign app? Any documentation about it?

When you have the integration installed, there was some sort of send for signature button added to the records.

We just rolled out Snap Sign for a client and they love it. They use it to collect signatures on Estimates. When the estimate is saved, it provides a button on the top of the screen to send for signature, similar to what @Ryan Longenecker said. Snap Sign's portal makes it easy to manage all documents that are in various phases of signature. It can be used to send email alerts when documents are opened and signed.