I'm sorry but JDownloader doesn't yet support HighDPI as this requires the LookAndFeel to support HighDPI as well and also requires more recent JVM version which would break backwards compatibility and also changes in GUI parts to support scaling/layout adapting.
At the moment there is only support to customize the font size via Settings-Advanded Settings, search for font size.
Please know that the GUI DOES NOT scale!
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JD-Dev & Server-Admin
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