Sometimes a customer may ask you to resend an invoice if they have mislaid it. You can find their invoices in two ways - via their customer page or the invoices page.
On the customer page you will find the invoices tab in the buttons opposite the name.
Clicking here will open a list of invoices for that customer only.
Alternatively find the invoice you need to send via the main invoice page. Use the filter to find the invoice by number or customer name.
Select the invoices using the check boxes and then press resend (found at the top) of the page.
You can add an email message and send.
The reminder feature works exactly the same way, however reminders can only be sent for overdue invoices. The only difference being the subject of the email will be "reminder for Invoice xxx".