private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    var location = @"c:\testform.pdf";

    pdfViewer1.LoadDocument(location);

    var rand = new Random();
            
    foreach (var field in pdfViewer1.Document.FormFill.InterForm.Fields)
    {
        if (field.FieldType == Patagames.Pdf.Enums.FormFieldTypes.FPDF_FORMFIELD_TEXTFIELD)
        {
            field.Value = $"This is a magical field: {rand.Next()}";
        }
    }

    // maybe some other modification... 
    // attach a file perhaps?

    pdfViewer1.Document.Save(location, Patagames.Pdf.Enums.SaveFlags.NoIncremental);
}