If you want to configure a project so that the same memo is added to each invoice, this is possible by adding a project rule 'Use memo on invoices'. This rule adds the project memo on all future invoices of this project automatically. However, in case of phased projects, you must add the memo on the project phases, not on the parent project.
You also have an option to set the 'Message on Invoice' on the Projects > General > Billing Options tab. Just enter a standard message to print on all invoices of the project. This message displays above the footer of the invoice. You can also set an invoice message at the global level (Global Settings > Billing).