Any installed JDownloader version is portable by default.
This means that it does not rely on registry entries or other dependencies.
For these reasons we do not provide a dedicated "portable version".
This means you can copy your JD install dir on any type of external disk (e.g. USB stick), plug it into another computer and start it from there.
Things to consider when using JDownloader in a portable way
For advanced users
All you really need for a fresh portable installation are the files JDownloader.jar and core.jar.
Copy them from your existing installation into a new folder.
On the first start via JDownloader.jar, JDownloader will fetch all other files via auto update.