How to set viewport width of a webpage

Question

I'm unable to set viewport width of webpage to generate PDF from URL.

My code is below:

 var pechkin = Factory.Create(new GlobalConfig()
    .SetPaperOrientation(true)
        .SetPaperSize(PaperKind.A2)
        .SetDocumentTitle("title")
        );
    var pdf = pechkin.Convert(new ObjectConfig()
                            .SetLoadImages(true)
                            .SetZoomFactor(1)
                            .SetPrintBackground(true)
                            .SetScreenMediaType(true)
                            .SetCreateExternalLinks(true)
                            .SetCreateInternalLinks(true)
                            .SetIntelligentShrinking(false)
                            .SetPageUri(urltxt.Text.Trim().ToString())
                            );
    #endregion

There is no such options to set viewport width.