If you have enabled the Notes Access option in the entity ‘detail form’ from the Form builder, the portal users can access the Notes to provide some information, details, or any instructions regarding the records (i.e., case details).
The portal users can add the Notes with attachments. The portal users can download the note attachment and delete the notes as per need.
Portal admin can access the notes from the CRM and manage them from there. The portal users will get the notes access option only on the details page of the record.