"DocuSignService • Services • pyConnect​" REST Service is responsible for handling the DocuSign events such as 'Completed/Signed/Declined.. etc..'.

This source activity, pxProcessDocuSignEvents, for REST Svc has access to extensible activities that you can override to alter the behaviour. 

#1. Parsing the PDF contents : look at #3rd step of pyProcessDocumentCompletedEvent​ activity. This is where you can get a hold of pdf contents as a string. See if you can parse as per your requirement.

#2. Get the custom/external fields : If you can trace and see the primary page for the Source activity of this svc, you should be able to know if the custom/external fields you are expecting are available as part  of the payload or not (Refer to step #1 of source activity). Or please refer to DocuSign documentation to see how you can make those external fields available on to the payload.

I have recently done this integration manually as the client is on version 7.1.8 and Docusign integration is available only for 7.2 onwards.

Using webhooks, where you liseen on an event has certain restriction. Please make sure you read through the following article from a security point of view.

https://www.docusign.com/blog/dsdev-securing-connect-webhook-listener/