foreach (FormField field in pdfDocument.AcroForm.FormFields)
{
    if (field is TextBoxField textBoxField)
    {
        List<TextFragment> oldTextFragments = new List<TextFragment>();
        foreach (VariableContentWidget widget in textBoxField.Widgets)
        {
            oldTextFragments.Add(widget.Content.NormalContentSource.Content.Where(f => f is TextFragment).Cast<TextFragment>().First());
        }

        textBoxField.Value = "New value";

        int index = 0;
        foreach (VariableContentWidget widget in textBoxField.Widgets)
        {
            widget.RecalculateContent();
            TextFragment newTextFragment = widget.Content.NormalContentSource.Content.Where(f => f is TextFragment).Cast<TextFragment>().First();

            TextFragment oldTextFragment = oldTextFragments[index++];
            newTextFragment.Font = oldTextFragment.Font;
            newTextFragment.FontSize = oldTextFragment.FontSize;
            newTextFragment.CharacterSpacing = oldTextFragment.CharacterSpacing;
            newTextFragment.Fill = oldTextFragment.Fill;
            newTextFragment.Stroke = oldTextFragment.Stroke;
            newTextFragment.StrokeThickness = oldTextFragment.StrokeThickness;
            newTextFragment.HorizontalScaling = oldTextFragment.HorizontalScaling;
            newTextFragment.Position = oldTextFragment.Position;
            newTextFragment.MiterLimit = oldTextFragment.MiterLimit;
        }
    }
}