Templates in Order Printer Emailer are built using HTML, CSS and Liquid.
In order to customize the default Shopify invoice design, you would need to edit the code for the template itself. You can do this on Step 1 of the setup wizard.
If you are not comfortable with code, then I can recommend my other app Order Printer Templates: https://apps.shopify.com/order-printer-templates
All of those designs are compatible with the Shopify Order Printer app (for printing and saving documents yourself) as well as with the free Order Printer Emailer app (for customer downloads).
It will allow you to customize the documents, without needing to mess with any code directly.