JAD is not a book file type it's an text base installation file that tells the Java Virtual Machine (JVM) on your phone the name of the executable (JAR) to download and install on your machine.
If you open up the JAD file in any plain text editor you will see the server location and the name of the JAR file to install. It's the "JAR" file that has the data you want. You should be able to open the "JAR" file with Winzip.
I'd recommend you forgo trying to convert the JAD/JAR and look for the equivalent PD here on this site of Project Gutenberg, Feedbooks, or Manybooks.net
The format you want for your Kindle is the MOBI format
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