Rename the file nav-virksomhet-loependeYtelser-v1-meldingsdefinisjon-1.2019.09.25-00.21-49b69f0625e0-javadoc.jar to nav-virksomhet-loependeYtelser-v1-meldingsdefinisjon-1.2019.09.25-00.21-49b69f0625e0-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder nav-virksomhet-loependeYtelser-v1-meldingsdefinisjon-1.2019.09.25-00.21-49b69f0625e0-javadoc
Double click index.html will open the index page on your default web browser.