Welcome to the Community, Adrian. It's either you'll manually send the created invoice PDF of your own company using the own email service or use third-party apps on the Apps page that can automate emails where reminders are sent through own email service.  I'll explain further with this.

The ability to attach a custom-created PDF instead of the default QuickBooks-generated invoice in reminders is not currently a feature due to the standardised processes designed to maintain uniformity and simplicity across the platform. However, looking for customised solutions can consider having QuickBooks with third-party apps in the Apps website that offer attaching custom PDF from your company to reminders.

You can also send us a Feedback to help developers enhance the functionality and tailor the software to better meet the needs. This feedback is important for addressing future feature developments like attaching the own created invoice PDF in reminders..

Once this is all through, you can now receive payments from your customers.

If there is any confusion regarding the workaround provided regarding with sending reminders using an own created invoice, kindly hit the Reply button so we can respond to you promptly.