Let's take a look and review the Online delivery options under the Sales setting. Make sure that you ticked the PDF Attached box to let them see the PDF file.
Sign in to QuickBooks Online and then select the Gear icon.
Choose Account and Setting, then go to Sales.
Under the Online delivery section, ensure to check the PDF Attached box.
Then you can test by using a dummy invoice and then send it to your email. If the issue persists, you can do the browser troubleshooting steps to isolate this. The browser application may have updates that need to refresh for it to load the PDF file attached to the invoice emails.