The PDF export functionality of the RadGantt uses non-Unicode fonts for the exported document. To overcome this and render Mandarin in the exported document, you will have to load a Unicode font, to set it as font-family for the RadGantt and to add it to its PDF export Fonts collection.
For your convenience, I prepared a runnable web site example, which you will find attached to my reply and modify to best fit in your specific scenario.
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