Using auto-create, I am creating a solicitation (Sourcing) and purchase order using an existing iProcurement Requisition.
Except the attachments, all the attributes / fields from iProcurement Requisition were flowing to both solicitation (Sourcing) and purchase order documents.
Could you please let me know the configuration steps, how I can make the attachments flow from iProcurement to Purchase Orders and Solicitations (Sourcing)

Your help is greatly appreciated.

Purchase order can be created from more than one requisitions.
For that reason the attachment at the requisition header is transferred over to the purchase order line (not at purchase order header). Please check PO lines and see if you are getting your attachments there.