I can walk you through with the steps on how to send invoices as PDF in QuickBooks Online.
In QuickBooks, there is an option to attach a downloadable PDF copy of your sales transaction. Perform these steps to enable this feature in the Account and Settings page:
However, if you're mailing them outside QuickBooks, download them manually by following the steps below:
Download a PDF copy in the Sales tab:
Create a PDF copy after creating an invoice:
That should do it. Check this article, to know more about emailing or printing invoices: How to email or print an invoice?
I'm a post away if you have more questions about QuickBooks. I'll be glad to help. Have a good one!