Under the Employee Center Roles, users can only see their own PO's. Is the a way to allow them to see all PO's and not only the ones created by them?

You can by creating a suitelet. By default they can only see po created by them or PO where they are approver but once approved and rejected they disappear from the list.

Thank you for the information. Never done a Suitelet. I might need to learn that first.