How to determine which form is open?

You wish to determine which form is open. Here is the code that will determine whether the form is open or not.
                    PDFViewer viewer = null;
                    foreach (Form OpenForm in Application.OpenForms)
                    {
                        if (OpenForm.GetType() == typeof(PDFViewer))
                        {
                            viewer = (PDFViewer)OpenForm;
                        }
                    }
                    viewer.CloseForm();