I installed Java JDK from here:
https://www.oracle.com/technetwork/java/javase/downloads/jdk13-downloads-5672538.html
and javadoc is now recognised from the terminal. However, if I run this:
javadoc -d .\application.windows64\source\*java
from the folder where my .pde file is located, I get the following error:
javadoc: error - No modules, packages or classes specified.
1 error
Thank you all for helping me.
Thomas