I think you should consider dropping the design part of the workflow - there are applications out there that are very good at this and it will be hard to compete there, and it is not part of your core product value. I believe the core value is in the dynamic PDF generation aspect - not the template design aspect.
Allow me to upload a pre created template that follows some sort of variable naming that is compatible with your API, and allow me to use just the API to generate documents.
Allow me to upload a pre created template that follows some sort of variable naming that is compatible with your API, and allow me to use just the API to generate documents.