private PdfForms _forms = null;
private bool _readOnly = false;

public MainWindow()
{
    InitializeComponent();
    PdfCommon.Initialize();
    pdfViewer1.DocumentLoaded += (s, e) => { _forms = pdfViewer1.Document.FormFill; };
}

bool ReadOnly
{
    get
    {
        return _readOnly;
    }
    set
    {
        if (_forms == null || _readOnly == value)
            return; //do nothing

        //Trick - Get access to internal propery - FormFill. 
        //To enable readonly mode we need to set that property to null.
        //And restore its original value (stored in _forms variable) to disable one.
        BindingFlags bindFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static;
        var propInfo = typeof(PdfDocument).GetProperty("FormFill", bindFlags);
        propInfo.SetValue(pdfViewer1.Document, value ? null : _forms, null);
        _readOnly = value;

        if(_readOnly)
            pdfViewer1.RenderFlags |= Patagames.Pdf.Enums.RenderFlags.FPDF_ANNOT;
        else
            pdfViewer1.RenderFlags ^= Patagames.Pdf.Enums.RenderFlags.FPDF_ANNOT;
    }
}