public static void SetAOI(IMxDocument mxd, double latitude, double longitude)
{
    UTM = SetUTMProjection(longitude);
    IPoint centerpoint = new PointClass();
    centerpoint.SpatialReference = NAD83;
    centerpoint.PutCoords(longitude, latitude);

    IEnvelope targetActiveViewEnv;
    targetActiveViewEnv = (mxd.FocusMap as IActiveView).Extent;
    targetActiveViewEnv.Project(NAD83);
    targetActiveViewEnv.CenterAt(centerpoint);
    (mxd.FocusMap as IActiveView).Extent = targetActiveViewEnv;
    mxd.FocusMap.SpatialReference = UTM;
}

public static void ExportMapPage(IMxDocument mxd, MapPage mapPage, string outputPath)
{
    IActiveView activeView = mxd.PageLayout as IActiveView;
    activeView.FocusMap.MapScale = mapPage.Scale; //6000
    mxd.ActiveView.PartialRefresh(esriViewDrawPhase.esriViewAll, null, activeView.Extent);
    string output = String.Format("{0}\{1}{2}", outputPath, "test", GetFileExtension(mapPage.FileFormat)); //PDF

    IExport exporter;
    switch (mapPage.FileFormat) {
        case FileFormatEnum.JPG:
            exporter = new ExportJPEGClass();
            break;
        default:
            exporter = new ExportPDFClass();
            break;
    }
    exporter.ExportFileName = output;
    exporter.Resolution = mapPage.DPI; //300

    IEnvelope pixelBox = new EnvelopeClass();
    double ratio = mapPage.Ratio;
    pixelBox.XMin = activeView.ExportFrame.left * ratio;
    pixelBox.XMax = activeView.ExportFrame.right * ratio;
    pixelBox.YMin = activeView.ExportFrame.top * ratio;
    pixelBox.YMax = activeView.ExportFrame.bottom * ratio;

    tagRECT exporterRectangle;
    exporterRectangle.left = activeView.ExportFrame.left * (int)ratio;
    exporterRectangle.right = activeView.ExportFrame.right * (int)ratio;
    exporterRectangle.top = activeView.ExportFrame.top * (int)ratio;
    exporterRectangle.bottom = activeView.ExportFrame.bottom * (int)ratio;

    exporter.PixelBounds = pixelBox;
    int hdc = exporter.StartExporting();
    activeView.Output(hdc, (int)exporter.Resolution, ref exporterRectangle, null, null);
    exporter.FinishExporting();
    exporter.Cleanup();
}