To view an Invoice, navigate to the Client Log for the Client in question and click the Invoice from the list.
From here, there is the option to mark the Invoice as paid by choosing a payment method from the drop down list. You are also able to email a copy of the Invoice to the Client (and any additional email addresses, using either the Email CC box or by separating each email address by a comma).
Emailing an Invoice
If you simply want to email a copy of the Invoice to the Client (or any other email address(es)) you can open the Invoice from the Client Log as described above, click to Email Invoice, check / enter the email address(es) and click Save.
This will not make any updates to the Invoice itself (unless of course you have chosen a payment method) and will schedule the email to be sent.