How do you attach a document in the purchase order? | SAP-mm

A document can be manually attached to a purchase order by using the document management I system in SAP.
In case when the purchase order is prepared using the T-code ME21N, then no attachment can be added.
You need to save the purchase order and re-open with the T-code| ME22N. Click the service for object button, select Service Object Button-> Created Create! Attachment, and then select the file to be attached.