Let me share additional information on how the invoice feature works.
The steps shared by my colleague, @MarsStephanieL, are for QuickBooks Online Simple Start, Essentials, Plus, and Advanced. Currently, the ability to add attachments to an invoice is not available in QuickBooks Self-Employed.
Deciding whether to upgrade your subscription depends on what really suits your business and the tax form you're filling. You can visit this page to see the features available on each QuickBooks version: QuickBooks Comparison Chart.