Could the NDA be sent by an attachment through the email?

Yes, you can send an NDA document through the email.
You can insert documents in your email by clicking the Insert Link drop-down in the Compose Email window. With this option, you can upload a PDF file to Vtiger CRM’s Document module and insert a link to the file in the email body. The documents open in the Document Viewer when a recipient clicks the link in the email body.

To learn more about Document Viewer in Vtiger CRM, click here.