Rename the file kie-server-services-task-assigning-user-system-7.74.1.Final-javadoc.jar to kie-server-services-task-assigning-user-system-7.74.1.Final-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder kie-server-services-task-assigning-user-system-7.74.1.Final-javadoc
Double click index.html will open the index page on your default web browser.