This is a generic question and I try to awnser.
1. We need to configure a content repository in OAC0. Content repository can be file system if you don't want to purchase one such as Filenet, Open text etc.
2. Create a document type using OAC2.
3. Assign the document type using OAC3 to a business object, in this case it is PO which is EKKO.
If you want to use Create Attachment button and want it to goto Content Repository, then
1. Create a new document category using OACT
2. Assign it to SOFFPHIO as New Category field in SKPR08
There could be more configuration steps.