Okay I now have an answer to this. When user performs Tools > Communicate, it invokes a POAPPRV workflow with root activity "RECEIVE_NOTIFY_REQUESTER_EVENT"
It then triggers two sub-workflows of POAPPRV type with root activity:
a) EMAIL_PO_PDF - To send the document to requesting user
b) NOTIFY_WEB_SUPPLIER_PROCESS - To send document to the suppliers
There are 2 ways to check now:
a) Check who email was sent to as part of EMAIL_PO_PDF workflow
b) (Better Approach) - Run Diagnostics Test "Workflow Item Details" for the POAPPRV workflow with NOTIFY_WEB_SUPPLIER_PROCESS root activity and locate the value of USER_ID attribute.
(You may get this from querying APPLSYS.wf_item_attribute_values as well)
Please mark as answered, if this resolved your problem.