How can I show additional text in tax-free PDF invoices in Magento?

Our Advanced Invoice Layout Extension for Magento allows you to have customer group specific texts on the PDF invoice document.
Therefore on the customer group configuration page in the Magento admin panel there is an additional text field where you can enter HTML-text that will be displayed on PDF invoices if the customer is contained in that customer group.

The automatic group assignment is a standard Magento feature which is explained in more detail in the Magento Documentation.