I am trying to change the scale of a map in an mxd OUTSIDE of ArcMap in a standalone Windows application. The problem is the scale is never persisted - the best code I have come up with is the following:

private void UpdateMapScaleInMxd(double scale, string mxdFullPath)
        {
            IMapDocument mapDocument = new MapDocumentClass();
            mapDocument.Open(mxdFullPath, "");
            IPageLayout pageLayout = (IPageLayout)mapDocument.ActiveView;

            IGraphicsContainer graphicsContainer = (IGraphicsContainer)pageLayout;
            graphicsContainer.Reset();
            IMapFrame mapFrame;

            IActiveView tmpActiveView = null;

            IElement element = graphicsContainer.Next();
            while (element != null)
            {
                if (element is IMapFrame)
                {
                    mapFrame = (IMapFrame)element;
                    tmpActiveView = (IActiveView)mapFrame.Map;

                    mapFrame.ExtentType = esriExtentTypeEnum.esriExtentScale;
                    mapFrame.MapScale = scale;
                    tmpActiveView.Refresh();
                }

                element = graphicsContainer.Next();
            }

            mapDocument.Save(false, false);
            mapDocument.Close();
        }

I check the mxd by opening it up in ArcMap and the map scale has changed, although the little combobox to change the scale is now greyed out - does anyone know what that means!?

So now I try to export the map as a PDF in code - and the most frustrating thing is - it exports the map at the previous scale that was set - NOT the scale I just changed it to! So infuriating - if anyone can help me understand where I am going wrong that would be great.