You can set up your QuickBooks account to email your invoice as a PDF. I can guide you on how to do it.
Click on the Gear icon.
Press on AccountsandSettings.
Refer to the Sales tab.
Tick on the pencil icon beside Onlinedelivery.
Select on the Showfulldetailsinemail radio button.
Make sure to put a check-mark on the PDFAttached box.
Hit on Save.
Tap on Done.
You can read through these resources for more details about the different file types that you can attach to any transactions and manage your sales forms: