Rename the file subscription-mongodb-spring-reactor-position-storage-0.16.6-javadoc.jar to subscription-mongodb-spring-reactor-position-storage-0.16.6-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder subscription-mongodb-spring-reactor-position-storage-0.16.6-javadoc
Double click index.html will open the index page on your default web browser.