If necessary, you can add a PDF version of an invoice to email notifications by enabling the setting "Send the PDF version of invoices in the attachments" on the Attachments page in the Admin area:
X-Cart 5.4.x - Store Setup > Email Notifications > Attachments
X-Cart 5.5.x - Settings > Email Notifications > Attachments
A PDF invoice will be added as an attachment to all order-related notifications.
You can extend the default PDF invoices feature with a paid PDF Invoices add-on that is more flexible and powerful.