GhostscriptPngDevice dev = new GhostscriptPngDevice(GhostscriptPngDeviceType.Png16m);
    dev.GraphicsAlphaBits = GhostscriptImageDeviceAlphaBits.V_4;
    dev.TextAlphaBits = GhostscriptImageDeviceAlphaBits.V_4;
    dev.ResolutionXY = new GhostscriptImageDeviceResolution(96, 96);
    dev.InputFiles.Add(@"E:\gss_test\indispensable.pdf");
    dev.Pdf.FirstPage = 2;
    dev.Pdf.LastPage = 4;
    dev.CustomSwitches.Add("-g<width in pixels>x<height in pixels>");
    dev.CustomSwitches.Add("-dFitPage");
    dev.OutputPath = @"E:\gss_test\output\indispensable_color_page_%03d.png";
    dev.Process();