Welcome to the Community, @mgyungner. I hope you're having a great week so far.
Is the Rental Agreement a PDF file or a saved image? If it's either of these, then you can upload the attachment into your Invoice with ease.
No worries, I'm here to walk you through the process:
There's also a section in your QuickBooks Online that you can upload attachments and add them to a list. I've provided the steps below on how to access this page:
That should do it! Let me know if you have additional questions. Have a great weekend.