如何在ItextSharp(PdfStamper)中增加PDF字段的字体大小？

using System;
using System.IO;
using iTextSharp.text.pdf;
using iTextSharp.text.pdf.interfaces;

public static string IncreaseFontSize(string inputPdf)
{
    string outputPdf = Path.GetTempFileName();
    using (PdfReader pdfReader = new PdfReader(inputPdf))
    {
        using (PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(outputPdf, FileMode.Create)))
        {
            AcroFields acroFields = pdfStamper.AcroFields;
            foreach (string fieldName in acroFields.Fields.Keys)
            {
                AcroFields.Item item = acroFields.GetFieldItem(fieldName);
                foreach (var field in item)
                {
                    PdfDictionary widget = field.GetWidget(0);
                    PdfDictionary ap = widget.GetAsDict(PdfName.AP);
                    if (ap != null)
                    {
                        PdfObject n = ap.Get(PdfName.N);
                        if (n is PdfIndirectReference)
                        {
                            n = ((PdfIndirectReference)n).GetRefersTo(true);
                        }
                        if (n is PdfDictionary)
                        {
                            PdfDictionary nAppearanceDict = (PdfDictionary)n;
                            PdfObject defaultAppearance = nAppearanceDict.Get(PdfName.DA);
                            if (defaultAppearance != null)
                            {
                                string da = defaultAppearance.ToString();
                                float fontSize = GetFontSize(da);
                                da = UpdateFontSize(da, fontSize * 1.5f);
                                nAppearanceDict.Put(PdfName.DA, new PdfString(da));
                            }
                        }
                    }
                }
            }
        }
    }
    return outputPdf;
}

private static float GetFontSize(string defaultAppearance)
{
    string[] parts = defaultAppearance.Split(' ');
    foreach (string part in parts)
    {
        if (part.StartsWith("Tf"))
        {
            string[] fontSizeParts = part.Split(' ');
            return float.Parse(fontSizeParts[1]);
        }
    }
    return 0;
}

private static string UpdateFontSize(string defaultAppearance, float newFontSize)
{
    string[] parts = defaultAppearance.Split(' ');
    for (int i = 0; i< parts.Length; i++)
    {
        if (parts[i].StartsWith("Tf"))
        {
            string[] fontSizeParts = parts[i].Split(' ');
            fontSizeParts[1] = newFontSize.ToString();
            parts[i] = string.Join(" ", fontSizeParts);
        }
    }
    return string.Join(" ", parts);
}

string inputPdf = "path/to/input.pdf";
string outputPdf = IncreaseFontSize(inputPdf);