If you have enabled the “Notes” access option in the detail page layout of any entity From 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 it from there. The portal users will get the notes access option only in the details page of the record.