Hi Pierre,
unfortunately forms has no API at the moment.
Also storing a "dummy" pdf on your own, will not meet the requirements for editing the pdf field with workflow.
I recommend to add this idea to our customer feedback forum, so others can vote and leave feedback:
https://docuware.uservoice.com/
Viele Grüße / With best regards,
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Gerardo Lisanti
Product Manager | DocuWare GmbH