Please note that PDF invoices created previously are not updated when you change the plugin’s settings in a latter stage, therefore, if you change your shop or document details (like the footer content), the previous invoices will not be affected, displaying nothing (if there was not a footer set at the moment of its generation) or the footer content was different.
In brief, we store both the document and shop settings per-order basis for historical reasons. For instance, if you change your shop address in the future, it should not affect old PDF invoices that you generated when your store was located in the old address.
That said, if you are just playing with the plugin configuration, or this is a new installation, you could try to activate the “Test mode” temporarily, under WooCommerce > PDF Invoices > General > Test mode. See: Show PDF documents with the latest settings.