The problem is that your font does not have glyphs for Arabic code points. You need to embed the font with Arabic glyphs such as arabtype.ttf.
string fontpath = Environment.GetEnvironmentVariable( "SystemRoot" ) + "\\fonts\\arabtype.ttf";
BaseFont basefont = BaseFont.CreateFont( fontpath, BaseFont.IDENTITY_H, BaseFont.EMBEDDED );
Font arabicFont = new Font( basefont, 10f, Font.NORMAL );
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