They should still be able to access the attachments from the PO Summary screen...if not here are 2 other things to try.
1.) Have the approver open the notification for the PO.
There should be some links at the bottom of the notification (be it the email or the notification from their Worklist in Apps).
Click on the View Document Details link.
This will take them to the PO and the attachments should show at the bottom of the page under the Attachments section.
*This only works for Header level attachments*
2.)If they click the Open Document link and have proper access to the PO form, this will take them directly to the PO form and you can access the attachments at any level
Thank you for the update. Yes agree with the options you have given. additionally I learned from a SR that if you update the document type as To Approver. this will allow the user to access the document from PO summary form. where as if the type is Document you can access it only from the PO form.