Invoice can be mailed to the customer for sharing the details or to remind him for the payment. to email Invoice follow the below process
Business Portal > Sales > Sales Order New > Invoice Selection > Email Invoice
Go to the invoice whose details are to be shared
Open the details of invoice and click on “Email Invoice” button
Select the reason for emailing and click on “Proceed” button
Enter the Email Id, Subject and the content for email and click on “Send”
The invoice will be sent to the requested mail