The option “Open in PDF viewer when done” is stored on the AutoCAD plotter configuration file (PC3). When you check or uncheck this checkbox, and hit OK, it will save the info back to the file DWG To PDF.pc3.
I don’t see the .NET API that can modify a PC3 file. We may try to look to import from ObjectARX SDK. The class PlotSettings and PlotConfig also do not have the “open file” option. We can use the PC3 file with method PlotSettingsValidator.SetPlotConfigurationName(plotSettings, plotConfigurationName, canonicalMediaName).
The plotConfigurationName is the name of PC3 file to use (plotConfigurationName = “DWG To PDF.pc3”). You can create another PC3 file copied from the original DWG To PDF.pc3 file with the saved unchecked “Open in PDF viewer when done” option.
Kean’s code presents how to plot a window using a PC3 file.