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 :
For sending Po through idoc some of the brief and important steps are as follows:
1)The configuration has to be done for sender and reciever systems initially.
2) The output type has to be created or can use the existing one where the medium is set to Distribution ALE.
3) Then the output type has to be assigned in the messages .
4) Once the document is saved the Po will be posted.
pls let me know if you want in more detail and reward if helpfull