One may attach any document to a PO manually without using the Document Management System in SAP, but no attachments can be added while you create a PO using ME21N. Save your PO and then open the PO using ME22N. There you can attach a document with the Service for Object button. Click the Service Object button –> Create –> Create Attachment then select your window directory, the file to be attached. This attachment is only for internal information. The system will not print this document automatically along with a PO printout.