Your clients can always view their invoices in their Client Area. You can also email them a direct link to the invoice by navigating to the shoot's Billing & Legal section and clicking the Email button on the far right side of the invoice.
But if you need to email them a PDF of the invoice, the easiest way to do this is to go to the invoice itself from the view of the Client Area. Navigate to the Shoot Overview page and click on the "View Client Area" link at the bottom right of the Additional Details section. Then click on the Billing & Legal section. Click the View button for the invoice. Alternatively, you can access an invoice by going to the shoot's Billing & Legal section and clicking the View button on the far right side of the invoice.
Once you are viewing the invoice, you can then use your computer's built-in Print to PDF functionality. If you're on a Mac, you can simply press Cmd+P to open the Print dialog and choose the "Mail PDF" option to easily create a PDF and open a new email with the PDF attached.
If you're on a Windows computer, there are several Print to PDF plugins freely available.
The Chrome browser has built-in print-to-PDF functionality. You could then email the saved PDF to your client.
Your clients also have the same ability to print-to-PDF in their Client Area.