Hi Mohit,
if your looking for custom fields read data from BBP_PDHSC and BBP_PDISC by passing
PO guid values and compare those values with signature values in BBP_DOC_CHANGE_BADI~BBP_PO_CHANGE.
OR Try Function module BBP_CHANGE_DOC_GET_NEW for getting the change documents.
Regards,
Venkata Krishna