Rename the file com.ibm.websphere.appserver.spi.opentracing.1.2-1.0.80-javadoc.jar to com.ibm.websphere.appserver.spi.opentracing.1.2-1.0.80-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder com.ibm.websphere.appserver.spi.opentracing.1.2-1.0.80-javadoc
Double click index.html will open the index page on your default web browser.