This might be a stupid question but I want to known your opinion (pros/cons) about that question:
After invoking a process via an email endpoint is it better to use The email endpoint default response or an Email Service (Foundation - Email - Send with Document) to send a generated PDF to the client ?
I would use the email endpoint default response if it was me. That way you don't have to worry about configuring an extra service in your process.
Also if the service is invoked using a different mechanism, let's say watch folder or the API, then it'll still work without modification. The service really works as a black box and can be called using any of the endpoints. This is a lot more reusable.