string headerFooterLocation = @"C:\HeaderFooterLoc.pdf";
string filePath = @"C:\testblah.pdf";           
            PdfDocument headerDocument = PdfReader.Open(headerFooterLocation, PdfDocumentOpenMode.Import);
            PdfPage headerPage = headerDocument.Pages[0];
            headerPage.CropBox = new PdfRectangle(new PointF(0, (float)headerDocument.Pages[0].Height - (float)yBegin.Point), new SizeF((float)headerDocument.Pages[0].Width, (float)headerDocument.Pages[0].Height));
            headerDocument.Save(filePath); // This document saved contains the correctly cropped stuff I want
           
            PdfPage blankPage = new PdfPage(outputDocument);
           
            XGraphics newGfx = XGraphics.FromPdfPage(blankPage, XGraphicsPdfPageOptions.Append);

            XPdfForm drawForm = XPdfForm.FromFile(filePath);
            newGfx.DrawImage(drawForm, 0, 0, drawForm.Width, yBegin.Point);

            outputDocument.AddPage(blankPage);
            string filename = output.Text;
            outputDocument.Save(filename);
            // ...and start a viewer.
            Process.Start(filename);