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Whenever you go to plot a drawing the plot window will show you the last used PC3 file.
How can I tell in .NET what was the last used PC3 file?
It's saved in the layout you're going to plot, here an article with example:
http://adndevblog.typepad.com/autocad/2012/07/gets-and-sets-the-current-plot-style-for-a-layout.html