Rename the file com.ibm.websphere.appserver.javax.persistence-2.0-23.0.0.8-javadoc.jar to com.ibm.websphere.appserver.javax.persistence-2.0-23.0.0.8-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder com.ibm.websphere.appserver.javax.persistence-2.0-23.0.0.8-javadoc
Double click index.html will open the index page on your default web browser.