In the Web Portal > CRM > Invoices > Edit pencil for the intended Invoice > View PDF > Email > email will pre populate with your default invoice email template (editable) > Send

Note: Be sure to check the Attachments section before you select Send, to make sure that any attachments you want to go with the Invoice are there. You can see the field when you scroll down, after the email 

Alternatively, you can also select the Share options within the Invoice, in order to share the link to the Invoice via email or SMS.

An invoice can be sent from the App > Accounts > Invoice > swipe right to select Email > Email will pre populate with your default invoice email template (editable) > Send

On your mobile device, you can also select SMS after swiping left, and the text message will pre-populate with the default Invoice SMS template (editable), which you can then send.