You can now choose to automatically attach the invoice, in the PDF format, for a transaction when the "payment successful" email is sent to your customers.
In your Chargebee site under Settings > Email Notifications > Enable Email Notifications, navigate to the "Payment Succeeded" email type and click "Customize Template".
In the "Template" page that you will be redirected to, scroll down and enable the "Invoices will be emailed as a PDF attachment" option as shown:
Enabling this option will automatically attach the invoice with all the "Payment Successful" emails that are sent out.