use iTextSharp.text.pdf.FontSelector;
iTextSharp.text.pdf.FontSelector selector = new iTextSharp.text.pdf.FontSelector();
// add 2 type of font to FontSelector
selector.AddFont(openSansfont);
selector.AddFont(chinesefont);
iTextSharp.text.Phrase phrase = selector.Process(yourTxt);
FontSelector will use the correct font for you!
Detailed Description from source file FontSelector.cs.
Selects the appropriate fonts that contain the glyphs needed to render text correctly. The fonts are checked in order until the character is found.
I forgot which order it search first!! please experience it!! Edit: the order is from the first addFont to the last addFont.