The official PDF receipt Mercurio gives you the moment your renewal application is signed and submitted. Contains the expediente (file) number, the submission timestamp, and the list of documents you uploaded. This is your proof that the renewal is officially 'en trámite'.
Issued automatically by Mercurio after you press 'Firmar y enviar' and AutoFirma confirms the signature. The download button appears on the confirmation screen — click it before closing the tab.