Quba Viewer, specifically designed for viewing e-invoices, is free. This simple open-source software solution for Mac OS, MS Windows, and Linux is available in English, French, and German. It allows to view XML-based invoices. Quba supports parts of Oasis UBL 2.1 and UN/CEFACTS Cross Industry Invoice, CII, 1.0/2016b, as well as Factur-X/ZUGFeRD, which embeds XML in PDF. It attempts to map the European standard EN16931.
