FormFieldTypes[] SupportedFieldTypes = { ... };

        public void GetPdfPageInfo(PdfDocument doc)
        {
            Stopwatch stopWatch = new Stopwatch();
            stopWatch.Start();

            var pageControls = doc.Pages.Select(p => doc.FormFill.InterForm.GetPageControls(p)).ToList();
            var count = 0;

            foreach (var pageControl in pageControls)
            {
                foreach (var control in pageControl)
                {
                    var fieldFullName = control.Field.FullName;

                    if (SupportedFieldTypes.Contains(control.Field.FieldType))
                    {
                        var field = control.Field;
                    }

                    count++;
                }
            }

            stopWatch.Stop();
            var ts = stopWatch.Elapsed;
            Debug.WriteLine($"{ts.Minutes:00}:{ts.Seconds:00}:{ts.Milliseconds:000}");  // about 7 secs
            Debug.WriteLine($"{count}");    // Count:728
        }