https://helpx.adobe.com/si/aem-forms/6-1/prepopulate-adaptive-form-fields.html#Thecuriouscaseofrepeatablepanels

It is quite easy to create custom components in OSGI. You basically implement an interface and use that code in the process step in your workflow J2 EE workflows can have input and output parameters and can be invoked quite easily from third party applicationsin OSGI workflows can be invoked using t...

Please work with your account manager to get a copy of the designer 

How are you generating the pdf?

How are you opening the pdf file?

https://experienceleague.adobe.com/docs/experience-manager-learn/forms/adaptive-forms/choice-group-items-adding-dynamically-article.html?lang=en#adaptive-forms

Have you tried using touch UI? 

https://swagger.io/docs/specification/basic-structure/

If you have the REST api you. An define the api using the swagger editor that  can be found onlineAEM forms supports swagger 2.0 

Yes http post is for submitting the pdf formWhat do you intend to do with the response you get from the REST call?have you considered using adaptive forms instead of pdf form