When user “Closed or Canceled” the purchase Order from Transaction–>Purchasing–>Edit Purchase Order those Purchase Order still show in Inquiry—>Purchasing—>Purchase Order Documents as Open PO .
Remove the Completed PO from Tools–>Routines—>Purchasing —> Removed the Completed Purchase Order.
Once the Completed Purchase Order Removed it won’t appear in Inquiry as Open Purchase Orders.
PS: Sometime I saw the fully Received Purchase Order still show in Purchase Order Inquiry as Open PO, Follow the above solution to resolve the issue.