When invoice reminders are enabled via Admin > Settings (Billing / Invoicing) > Enable Invoice Reminders (ON)
A reminder will be sent for unpaid current invoices on the date due and then at 30-day intervals until the invoice has been paid or a new invoice has been generated. Email reminders are only sent to clients whose invoice was initially emailed. It does not send reminders to clients whose invoices were downloaded and mailed to the client.
Invoice reminders are visible in the activity timeline by selecting the Invoice to view details. It will only send if it is the MOST RECENT open invoice so if the client has >1 invoice, it won't send them both, only the latest(the most recent unpaid invoice).