If you were to create this PO with no attachments and decided at a later point that you would like to upload an attachment then this can then be uploaded and removed at any point by going to the 'manage appended attachments' function. If you cannot see this option then you will need to make sure you have the option enabled in your user settings by going to Admin > Users and checking the option 'can append attachments'.
This is also the case for amended PO's, if a PO was amended and an attachment was uploaded during the amend process then the only way to remove the attachment would be to go through the amend process again and remove the attachment.
If a PO was amended and the attachment was removed during the amending process and at a later point you wanted to upload an attachment you can go to the 'manage appended attachments' option and upload an attachment and also remove it at any point.