How to open Javadoc JAR file in web browser
- Rename the file github-com-ckeditor-ckeditor5-39.0.1-javadoc.jar to github-com-ckeditor-ckeditor5-39.0.1-javadoc.zip
- Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder github-com-ckeditor-ckeditor5-39.0.1-javadoc
- Double click index.html will open the index page on your default web browser.
How to generate Javadoc from a source JAR?
Running the command javadoc:
javadoc --ignore-source-errors -encoding UTF-8 -sourcepath "github-com-ckeditor-ckeditor5-39.0.1-sources.jar" -d "github-com-ckeditor-ckeditor5-39.0.1-javadoc" -subpackages 
