使用itextsharp(或任何c#pdf库),如何打开PDF,替换一些文本,然后再次保存？

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

namespace ReplacePDFText
{
    class Program
    {
        static void Main(string[] args)
        {
            string inputFile = "input.pdf";
            string outputFile = "output.pdf";
            string oldText = "old_text";
            string newText = "new_text";

            PdfReader reader = new PdfReader(inputFile);
            using (PdfStamper stamper = new PdfStamper(reader, new FileStream(outputFile, FileMode.Create)))
            {
                for (int i = 1; i <= reader.NumberOfPages; i++)
                {
                    PdfDictionary pageDict = reader.GetPageN(i);
                    PRIndirectReference pageRef = (PRIndirectReference)pageDict;
                    PRStream contentStream = (PRStream)pageRef.GetRefersTo(reader);
                    byte[] contentBytes = PdfReader.GetStreamBytes(contentStream);
                    string content = System.Text.Encoding.UTF8.GetString(contentBytes);

                    content = content.Replace(oldText, newText);

                    PRStream newContentStream = new PRStream(reader, contentBytes);
                    pageDict.Put(PdfName.CONTENTS, newContentStream);
                }
            }
        }
    }
}