Automatic reminders for outstanding invoices are turned on by default.  To turn them off for a given project, go to that project's setting page, and uncheck this box at the bottom of the page.
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Doing this will update the settings for all existing invoice on the project as well as invoices created in the future for that particular project.
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With outstanding invoices, proposals and contracts, you have the option to send reminders manually. Navigate to the specific document that was previously sent to your client, and locate the reminder option in the top righthand corner.