Attaching a PDF receipt to an invoice email is unavailable, tmitchell2. Let's send this request to our Product Developers so they can consider adding this option to the customization setting.
On the Customize panel of your Invoice, the PDF dropdown menu is only for the Pay Invoice Link. Meanwhile, the Email section allows you to personalize the message your customers see with their Invoices. Therefore, we're unable to add a receipt to these customization options.

Additionally, I acknowledge the convenience of attaching or linking the receipt when emailing invoices for faster transactions. With this, we can select the Feedback button on the Invoice page to submit a suggestion to our product developers.

Keep an eye on your suggestions on this website and visit our blog for recent developments.
Moreover, consider setting up online payments so customers can pay their obligations electronically and efficiently. If interested, refer to Step 2 and scroll down to Turn on payments in this article: Create invoices in QuickBooks Self-Employed.
Furthermore, check out this guide to record your money in transactions in the program: Connect bank and credit card accounts to QuickBooks Self-Employed.
Your feedback is invaluable as it helps us improve our services and meet your needs. We're here to support you and make your experience as convenient as possible.