Rename the file izumi-reflect-thirdparty-boopickle-shaded_native0.4_2.11-2.3.8-javadoc.jar to izumi-reflect-thirdparty-boopickle-shaded_native0.4_2.11-2.3.8-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder izumi-reflect-thirdparty-boopickle-shaded_native0.4_2.11-2.3.8-javadoc
Double click index.html will open the index page on your default web browser.