HELP NEEDED! Saving form with user input

[webstinction]

I've spent 3 days now trying to find out how to save a PDF form that has some fields pre-populated by pdf-lib (which works great) and then load that byte array into an iframe src. The form is displayed, and the user can add further data (checkmarks, text, etc). I can click on the 'Save with changes' on the toolbar, but I want to programmatically save the document (and hide the toolbar) as part of my app. If I save using the iframe src, it doesn't save any user changes.

My question is, how can I programmatically save the pdf data with user changes? If I can get the base64 data from the iframe embed object I can work with it, but I just can't work out how! $$$ on the line for anyone that gives me an answer that works!