Your clients can view and print their invoices in their client area. However, if you would like to email them an invoice, navigate to the Billing & Legal section of the shoot and click the "Email" button next to a payment. This will take you to an email page where you can send a link to the invoice to your client.
Alternatively, you can also email an invoice from the invoice itself. Navigate to the Billing & Legal section of the shoot, and click on the "View" button on the far right side of an invoice payment. In the upper left hand corner of the invoice there is an "Email Invoice" button you can click to send it to your client.