public static double PixelsToUnits(int pixels)
{
	var flags = BindingFlags.NonPublic | BindingFlags.Static;
	var dpiProperty = typeof(SystemParameters).GetProperty("Dpi", flags);
	double Dpi = (double)(int)dpiProperty.GetValue(null, null);
	double PixelSize = Dpi / 96;
	return (double)pixels / PixelSize;
}

private void btnSetCurrentPage_Click(object sender, RoutedEventArgs e)
{
	var rectInPixels = pdfViewer1.GetSelectedRects(0)[0];
	var rectInWpfUnits = new Rect(
		PixelsToUnits(rectInPixels.X),
		PixelsToUnits(rectInPixels.Y),
		PixelsToUnits(rectInPixels.Width),
		PixelsToUnits(rectInPixels.Height));
	var pagePt1 = pdfViewer1.ClientToPage(0, new Point(rectInWpfUnits.X, rectInWpfUnits.Y));
	var pagePt2 = pdfViewer1.ClientToPage(0, new Point(rectInWpfUnits.X + rectInWpfUnits.Width, rectInWpfUnits.Y + rectInWpfUnits.Height));
	var pageRect = new FS_RECTF(
		Math.Min(pagePt1.X, pagePt2.X), //left
		Math.Max(pagePt1.Y, pagePt2.Y), //top
		Math.Max(pagePt1.X, pagePt2.X), //right
		Math.Min(pagePt1.Y, pagePt2.Y) //bottom
		);
	var annot = new PdfSquareAnnotation(pdfViewer1.Document.Pages[0], pageRect, FS_COLOR.Green, FS_COLOR.Blue);
	annot.GenerateAppearance(Patagames.Pdf.Enums.AppearanceStreamModes.Normal);
	if (pdfViewer1.Document.Pages[0].Annots == null)
		pdfViewer1.Document.Pages[0].CreateAnnotations();
	pdfViewer1.Document.Pages[0].Annots.Add(annot);
	pdfViewer1.Document.Pages[0].Dispose();
	pdfViewer1.InvalidateVisual();
}