Quba is a invoice viewer supporting Peppol-, CII-, UBL-, XRechnung- and Factur-X/ZUGFeRD.
Through sponsorship of 
The XML part of hybrid invoices and orders is now also shown immediately next to the PDF-view.
The removal of the "extra click" which was so far neccessary for this purpose has been removed as part of the preparation for the planned mandatory german domestic B2B-e-invoicing.
Quba is open source, can also validate invoices and orders using Mustangserver when online and can be downloaded free of charge for Linux, Windows and Mac.