Rename the file org.wso2.carbon.identity.oauth2.token.handler.clientauth.mutualtls-2.4.25-javadoc.jar to org.wso2.carbon.identity.oauth2.token.handler.clientauth.mutualtls-2.4.25-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder org.wso2.carbon.identity.oauth2.token.handler.clientauth.mutualtls-2.4.25-javadoc
Double click index.html will open the index page on your default web browser.