Various partners from the private and administrative sectors have developed in collaboration with FeRD a data format called ZUGFeRD. The ZUGFeRD approach builds on a combination of PDF with integrated structured data. The receiver decides if he processes structured data or just prints out the PDF, following the traditional manual process. ZUGFeRD is based on a core invoice, which leads to easy implementation. As it based on standards (XML, PDF/A), tools are available.
You can find the specification of ZUGFeRD 1.0 here.