Help, to set all formfield readonly in pdf

Not a hard thing.Here is an article exactly helps you ......

How to set read-only fields on a form.

Just download this free PDF API ,add the reference to your project and add the following code .

PdfDocument doc = new PdfDocument();
doc.LoadFromFile("FormField.pdf");

PdfFormWidget widget = doc.Form as PdfFormWidget;

for (int i = 0; i < widget.FieldsWidget.List.Count; i++)
   {
     PdfField f = widget.FieldsWidget.List[i] as PdfField;
     if (f.Name == "username")
        {
           PdfTextBoxFieldWidget textboxField = f as PdfTextBoxFieldWidget;
           textboxField.Text = "Sky.Luo";
        }
           f.Flatten = true;
           f.ReadOnly = true;
   }
doc.SaveToFile("FormFieldEdit.pdf");
System.Diagnostics.Process.Start("FormFieldEdit.pdf");