Go to Sales and then click on Sales Orders.

Select the Sales Order that you wish to email and then click on the right arrow (other commands) that is above the table headings for the sales order table. This should give you a drop down menu and two of the choices will be "email client invoice PDF" and "email client quote PDF."

Click on the option you wish to execute and you should then receive a message that PDF has been emailed to the client.

Note. For the Invoice to be sent, you must set the default payment type for the client you are trying to invoice.