How to save a PDF as read-only / flattened?

Question

I'm using PDFSharp to generate a PDF document with fields filled in. When the doc is saved, I'd like it to be read-only, aka flattened. I've tried the below, but still, when opening the PDF in Adobe, the fields are editable.

   using (PdfDocument form = PdfReader.Open(outputFormLocation , PdfDocumentOpenMode.Modify))
        {
            //do stuff...
            //Save
            PdfSecuritySettings securitySettings = form.SecuritySettings;
            securitySettings.PermitFormsFill = false;
            securitySettings.PermitModifyDocument = false;
            securitySettings.PermitPrint = true;

            form.Save(outputFormLocation);

Answer 1

Setting all fields' ReadOnly property works for me using PdfSharp 1.32, using PdfSharp.Pdf.AcroForms (this may not have been available at the time the question was posted). For example:

PdfDocument document = PdfReader.Open("file.pdf", PdfDocumentOpenMode.Modify);
PdfAcroForm form = document.AcroForm;
PdfAcroField.PdfAcroFieldCollection fields = form.Fields;
string[] names = fields.Names;

for (int idx = 0; idx < names.Length; idx++)
{
    string fqName = names[idx];
    PdfAcroField field = fields[fqName];
    PdfTextField txtField;

    if ((txtField = field as PdfTextField) != null)
    {
        txtField.ReadOnly = true;
    }
}
document.Save("file.pdf");