Hello there, pm5395a-gmail-co.
Currently, QuickBooks Self-Employed doesn't have an option to add attachments when creating invoices. Though, this option is available in QuickBooks Online: Attachments in QuickBooks Online.
We'll take note and let our product development team know about this option for QBSE. They might put it into consideration for product updates.
Meanwhile, you can view these links for any updates regarding the program:
We'll keep these links updates so you'll know what are the changes and new in the program
Let us know if you need anything else. Have a great day!