kZoomToExtentsMode determines whether the exported drawing is in the Zoom to Extents mode. If enabled, the layout geometry is zoomed to the paper size, otherwise layout settings are applied.
So, to export a part of a drawing, you need to unset this flag and set settings relating to plot area in PlotSetting. Please refer to this blog article: Exporting part of a CAD drawing to a .pdf file. Also this article from may be useful: Export to PDF Parameters and Flags.