For sending IDOC for Purchase order you will use idoc

For sending IDOC for Purchase order you will use idoc
Message type : ORDERS
Basic Type : ORDERS05
Create one RFC Destination in SM59 and give that entry in Port in WE21.
Create Partner profile in WE20 and Port for receiver system in WE21.
Use your port in Partner profile in WE20.
In Partner Profile create one entry for Message type ORDERS as Outbound.
In your tcode when you are creating Purchase order set output message in your header condition for Outbound to send that purchase order to that Partner Profile.
Check link :
http://www.saptechnical.com/Tutorials/ALE/AutoIDOCGen/AutomaticIDocGeneration1.htm