Hi Chitturi,
You could also display the information of the Billing document under the Approve / Reject Button, when the workitem is executed.
Create a new standard task by copying the Generic decision task (TS00008267) and enter the information which you want to display in the "Description" Tab. You could make use of the Task container to populate dynamic texts.
Configure this created standard task in the User Decision which you have created under Control Tab.
The description will be displayed as below (for example)
Thanks,
Karthikeyan