Is there a way to get the PDF Services API webhook CALLBACK directly back into originating App?

I am new to using the Webhook CALLBACK feature and I am wondering if it is possible to have the HTTP request in my Azure Logic App, wait for the CALLBACK so that i can continue processing the file.
For example, I am looping over an array of images and converting to PDF's.  I then need to combine the results into a single doument using Merge.  Setting up another App to recieve the CALLBACK is not really a convenient option.   I have been polling the endpoint but I thought the CALLBACK was supposed to eliminate polling.