Hi Harish,
This would be complicated.
In high level, one possible solution i can think of, others than involving adobe form and workflow (you need this because workflow to route for approval), make use of web services in SAP .
Expose your RFC FM as web service.
In adobe form, create Submit button to trigger web service (RFC FM) by passing form data and initiate workflow events.
Workflow to generate work item with to adobe form to correspond approver
Approver receive the work item / adobe form. To keep this simple, I would suggest to use some flag to control visibility of Submit / Approve / Reject button. Approve / Reject button should only available at this step.
When approve / reject button is clicked, trigger web service again by passing the approve/reject status back to workflow by triggering workflow event in RFC FM.
In your workflow, you can do further post processing like send rejection / approved notification back to requester or perform data update into database; up to your requirement.
Below are some reference for related your case
Trigger workflow event
http://scn.sap.com/thread/1651325
Create adobe as attachment in work item
http://scn.sap.com/thread/1440102
RFC FM as web service
http://help.sap.com/saphelp_nw04/helpdata/en/e9/ae1b9a5d2cef4ea4b579f19d902871/content.htm
adobe form & web service sample.
Hope this helps.
regards,
Xiang Li