WHMCS generated invoices are neat and professional, though sometimes it is useful to be able to add or edit information to/on these PDF invoices. The file that needs to be edited is named invoicepdf.tpl and is located in the following directory:
WHMCS_DIRECTORY/templates/THEME_NAME/
The easiest way to edit this file is to do so through SSH. To enable SSH for cPanel you can read this guide. Alternatively, you can edit this file by logging into cPanel and going to File Manager, click on public_html on the left and then locate invoicepdf.tpl in the directory referred to above.