Rename the file com.liferay.portal.osgi.web.portlet.container.test.util-2.0.13-javadoc.jar to com.liferay.portal.osgi.web.portlet.container.test.util-2.0.13-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder com.liferay.portal.osgi.web.portlet.container.test.util-2.0.13-javadoc
Double click index.html will open the index page on your default web browser.