Step 1: Click on Billing on the left-hand navigation menu and then open any invoice from the Invoice table by clicking on the invoice ID.
Step 2: Once the invoice is open, scroll slightly down to see the 'Add Notes' section.
Step 3: In the text box, enter the note required.
Step 4: Click on the Save button to proceed and see confirmation of successful note addition.
Step 5: Now, you can download this invoice using the Download button or email this invoice to the customer using the Send E-mail button. The added note will be visible on the downloaded invoice pdf document as well as on the invoice emailed to the customer.