Rename the file org.wso2.carbon.extension.identity.authenticator.emailotp.feature-3.0.26-javadoc.jar to org.wso2.carbon.extension.identity.authenticator.emailotp.feature-3.0.26-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder org.wso2.carbon.extension.identity.authenticator.emailotp.feature-3.0.26-javadoc
Double click index.html will open the index page on your default web browser.