Hi Ralph, due to XSS protection, cross browser restrictions it's not possible to edit the content of an embedded iframe using JavaScript. This is a security measure implemented by the browser. The approach you are considering isn't possible.
What you could do is integrate the Dropbox sdk and allow your users to upload to Dropbox via your knack form. This wouldn't require any sort of embedded element, and would also be very reliable. With the this method you would be able to save the file with the required order information in a manner that is seamless with your application and the end user wouldn't even notice.
If you have any questions you can contact me:
Kelson@ksensetech.com
https://www.ksensetech.com/