Step 1 : Navigate to clients widget in the top menu.

Step 2 : Scroll down to invoices section where you will see list of all the paid and unpaid invoices.

Step 3 : Click on the send email icon to view the email editor.

Step 4 : You will see the invoice attached to the email already. You can also add other files from your pc by clicking on "Upload Files".

Step 5 : Click on "Send" button and see the invoice sent to client successfully via a toaster message.

Reference