Is it possible to add additional documents to a template?
Adding an extra document is something not supported. Although it is possible indeed, it requires advanced PHP skills since it is quite complex.
That said, if you have a valid license for either the Professional extension or Premium Templates, we can send you a extension, free of charge, which adds an extra PDF document which you can edit by yourself (do not that we do not offer extra support on this).
Can they also be customized with the customizer or will they be “static” (like the free template/documents).
The Premium Templates extension includes a customizer which allows you to edit the PDF document layout, like adding or removing columns in the order table or adding/removing rows from the totals’ area. In addition, you can use conditional custom blocks to add custom details. However, if you need to apply advanced layout customization, you could use a custom PDF template.
Please note that, if you have further questions, I have to ask you write us to [email protected], since WordPress.org does not allow us to provide support or answering pre-sale questions for paid plugins here.