You can email an invoice at any time by opening an existing invoice and clicking on the Email button.
The fields in the Email window will be populated automatically, but can be edited as required.
Using email templates is a great way to speed up creating and sending emails. Please see Email Templates for more information.
Sending an Online Invoice Link in the email of the body is highly recommended as not all users have PDF reading software. There is also enhanced PayPal integration functionality available in online invoices via the settings in the Invoice Brands and Templates.