<table border="1">

 var fieldsList = formFields.ToList();
 for (int i = 0; i < fieldsList.Count; i++)
 {
     var field = fieldsList[i];
     bool flattenCurrentField = true;
     if (field.FieldType == FormFieldType.PushButton)
     {
         PushButtonField pushButtonField = (PushButtonField)field;
         PushButtonWidget[] widgets = pushButtonField.Widgets.ToArray();

         foreach (var widget in widgets)
         {
             var baseType = typeof(PushButtonWidget).BaseType.BaseType.BaseType;
             PropertyInfo[] properties = baseType.GetProperties(BindingFlags.NonPublic | BindingFlags.Instance);
             PropertyInfo isHidden = properties.FirstOrDefault(p => p.Name == "IsHidden");
             bool value = (bool)isHidden.GetValue(widget);
             if (value == true)
             {
                 flattenCurrentField = false;
                 break;
             }
         }
     }

     if (flattenCurrentField)
     {
         document.AcroForm.FlattenFormField(field);
     }
 }

RtfFormatProvider provider = new RtfFormatProvider();
RadFlowDocument document = provider.Import(File.ReadAllText(@"..\..\test.rtf"));
 
PdfFormatProvider pdfProvider = new PdfFormatProvider();
var tables = document.EnumerateChildrenOfType<Table>();

foreach (var table in tables)
{
    table.Borders = new TableBorders(new Border(BorderStyle.None));
}


using (FileStream stream = File.OpenWrite(@"..\..\result.pdf"))
{
    pdfProvider.Export(document, stream);
}