VX12N - attach a PDF to Financial document

Hello,

We have a requirement to attach a PDF copy to Financial document for Letter of Credit (T-code VX12N --> system --> Services for Object)

I searched more about this online, but everywhere I find solution to attach a document to Sales Orders using Generic Object Service (GOP). We already have this functionality enabled.

There were few forums with same question, but no one provided answer how to enable services of object for Financial document or even other possible ways to achieve this requirement.

I also tried to configured GOP for Financial document. But I was unable to find standard object type for Financial document. (T-code OAC3).

Reference link for GOP - https://help.sap.com/saphelp_46c/helpdata/en/87/a5edf8dd4511d289840000e8216438/content.htm?no_cache=true

Any idea if it is even possible to create a custom object type for Financial document that enables Service for Object functionality? If so can you please provide steps?

Thanks,

NSoni