Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window nav-virksomhet-dokumentnotifikasjon-v1-meldingsdefinisjon-1.2019.01.08-09.21.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.