You can attach manually any document to PO without using Document Management System ( DMS ) in SAP 4.6C.
But you cannot attach document while you create PO in T/Code ME21N
Save you PO ( ME21N ) and again open with change ( ME22N ) then you can attach document through Service for Object button. Service for Object button available down side of Command screen ( in your PO screen ME22N )
Click your Service Object button -> Create -> Create attachment
then select your window directory ( which file you need to attach) select your file Now your system shows message Service "Create attachment" is started.
Save again your PO. You can see (open) your attachment in same field ( Service for Object ) select and Create and check attachment list. You can attach as many document in your PO.
Note: This is only information for internal or you can print separately this document for sending to vendor ( fax ) System will not print automatically along PO print out.