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I can tell what happens to the PDF attachment on an invoice, dperez3.
We've been alerted about this behavior. And, Product Engineers are looking into this to determine why attachments are not being sent with emails.
For now, you can turn the settings back on. Then, manually download the estimate and send it to your client outside QuickBooks.
Let me show you how:
Check out this guide to learn how to create and send invoices in QuickBooks Online.
Let me know if you need anything else aside from creating estimates. I'll be around to help you.