Once your document is saved, you can send it the client by clicking on the Send to Client button.
A popup will be displayed with an email field and an optional message to include in the email your client will receive.
The email field is prepolulated by the client's email field, but you can override it now if you need to.
The optional message will be included in the email's content.
Alternatively, you can send the invoice yourself, and then manually change the Invoice status to Sent.