You can email approved invoices in three ways:

• Auto send email as part of Invoice Runs
• Send email manually from the Invoice Details page
• Schedule a batch job

You can automatically send approved invoices through email as part of the Invoice Runs. However, if you want to manually email an invoice in your preferred format, you can do so from the Invoice Details page.

If there are a lot of invoices that you want to send through email which were somehow not sent via the Invoice Runs, you can schedule a batch job instead; please refer Schedule Invoice Email Delivery. Check out Invoice Runs- Processing Option to know how you can auto send Invoice Email.