internal static Matrix CalculateRotatedPageTransformationInDip(RadFixedPage page)
{
    Rect visibleContentBox = new Rect(page.Size);

    double rotationAngle = 0;
    var rotationCenter = new System.Windows.Point();

    switch (page.Rotation)
    {
        case Rotation.Rotate90:
            rotationAngle = 90;
            rotationCenter = new System.Windows.Point(visibleContentBox.Height / 2, visibleContentBox.Height / 2);
            break;
        case Rotation.Rotate180:
            rotationAngle = 180;
            rotationCenter = new System.Windows.Point(visibleContentBox.Width / 2, visibleContentBox.Height / 2);
            break;
        case Rotation.Rotate270:
            rotationAngle = 270;
            rotationCenter = new System.Windows.Point(visibleContentBox.Width / 2, visibleContentBox.Width / 2);
            break;
        default:
            rotationAngle = 0;
            rotationCenter = new System.Windows.Point();
            break;
    }

    Matrix matrix = new Matrix();
    matrix.RotateAt(rotationAngle, rotationCenter.X, rotationCenter.Y);

    return matrix;
}
public Matrix InverseMatrix(Matrix m)
{
    var det = CalculateDeterminant(m);
    var det1 = 1 / det;
    return new Matrix(m.M22 * det1, -m.M12 * det1, -m.M21 * det1, m.M11 * det1, ((m.M21 * m.OffsetY) - (m.OffsetX * m.M22)) * det1, ((m.OffsetX * m.M12) - (m.M11 * m.OffsetY)) * det1);
}

public double CalculateDeterminant(Matrix m)
{
    return (m.M11 * m.M22) - (m.M12 * m.M21);
}

if (page.Rotation == Telerik.Windows.Documents.Fixed.Model.Data.Rotation.Rotate270)
{  
    var m = CalculateRotatedPageTransformationInDip(page);
    var matrix = new Matrix(1, 0, 0, 1, pointInDoc.X, pointInDoc.Y) * InverseMatrix(m);
    editor.Position = new MatrixPosition(matrix);
}