JDownloader can be free, because we offer additional software during the installation process.
This means, that there is an extra screen during the installation that explains the offer, and gives you the option to uncheck it. In this case, no 3rd party software will be installed.
If you installed the offer by accident, I'm afraid that you missed this screen, and clicked "continue". I'm sorry about this.
You can uninstall the installed additional software in your Windows Software Control Panel, or by using a tool like this: https://www.chip.de/downloads/AdwCleaner_58118522.html
In case you prefer an installer that does not offer and install any additional software, please use our adware-free installer: https://jdownloader.org/jdownloader2
If you need any further help with installing or uninstalling, please do not hesitate to contact our support.