You can adjust the size of the page margins on your invoice template. However, the bottom part is working as design that it won't move up and down when you print and preview the form.
I'll take note and pass along this feedback to our Product Team so they can discuss how to update the bottom margins.
As a workaround, you can import a personalize invoice template with a footer, then import it to QuickBooks. Just make sure the bottom margin is already customized since you won't be able to edit it in the system once imported. You'd need to re-import a new one if you want to change the page margins.