How can I change the header of the pdf sent?

Hi,
We send only receipts and not invoices that’s why we set the receipts in the backend and customize all the fields (n. receipt, n. order, logo, etc.) and it works but when the customer receive the pdf I still read in the header (below the logo) “invoice” instead than “receipts”.

In addition we see correctly “receipt” in the account profile when users generate it, we also see the change correctly in the order list when we generate the pdf.

How can I change the header of the pdf sent?

Thanks in advance!
Ros