To add a custom text to for example an invoice, you must create a custom report (go to Setup > Reports). Click the button “Create”.
LEARN MORE: Edit your reports to match your company.
In the “Create report layout” dialog, fill in a report name (like “Our invoice”) and pick the group “Invoice”.
Now we are ready to add your custom text. In the box “Extra information”, switch to edit mode. Now you can add your text in the column “Default value”. In the example view, you can see a preview of your custom text.