Rename the file jeaf-generator-test-open-api-internal-service-objects-1.11.7-javadoc.jar to jeaf-generator-test-open-api-internal-service-objects-1.11.7-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder jeaf-generator-test-open-api-internal-service-objects-1.11.7-javadoc
Double click index.html will open the index page on your default web browser.
