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Hi there, @HeartsAndMinds. I see how important to let the customer view the estimate right away. I've got steps you can consider performing.
Let's make sure that your estimate is set to PDF. To check this you can follow the steps below:
- Click the Gear icon, and then choose Account and Settings.
- Select the Sales tab, and then navigate through the Online delivery section.
- Make sure to check the PDF Attached box, and then choose Online invoice from the drop-down.
- Click Save, and then Done when you're ready.
See the screenshot below for your reference. 
Then resend the estimate, if the issue persists, we can go ahead and let your customers open the invoice in a private window. This type of window doesn’t store any cache that causes a browser to load slowly or experience unusual behavior.
- Google Chrome: Ctrl + Shift + N
- Mozilla Firefox: Ctrl + Shift + P
- Microsoft Edge: Ctrl + Shift + P
- Safari: Command + Shift + N
If they are able to view the estimate, ask them to go back to their regular browser and clear its cache to prevent any unexpected behavior. They can also use other compatible browsers in QBO.
To learn more about how the estimate feature works in QuickBooks Online, see this article: Set up and use estimates.
For future reference, you may find below articles helpful:
- Copy an estimate to a purchase order
- Customize invoices, estimates, and sales receipts in QuickBooks Online
You can count on me if you have more questions about managing your estimates in QuickBooks. I'd be glad to help you some more.
