Hello Ulrich,
Thank for your reply, the file currently exists in SAP system, we are working in the external system using JCO to fetch the file. As per our understanding we are not receiving a PDF but an OTF which we suppose to convert into a PDF. (as per sandra in the post above looks like they should be able to send directly a PDF. We are currently reviewing this also.)
the function module being used is a custom one, the name is ZBAPI_CUSTOMER_AR_STATEMENT.
I'm attaching the trace content here, like they said the content should be OTF as per client.