看官方文档，已经停止更新了，进入了维护阶段，推荐使用iText7。 https://github.com/itext/itext7-dotnet

使用的版本为

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="BouncyCastle" version="1.8.6.1" targetFramework="net48" />
  <package id="iTextSharp" version="5.5.13.2" targetFramework="net48" />
  <package id="itextsharp.xmlworker" version="5.5.13.2" targetFramework="net48" />
</packages>
代码示例，测试文件请自己准备：
    class Program
        static void Main(string[] args)
            FileStream fileStream = new FileStream(@"C:\Data\pdftest.pdf", FileMode.Open, FileAccess.Read);
            var fileBytes = CopyPagesFromPDF(fileStream, new List<int> { 2, 10, 8 });
            FileStream destFileStream = new FileStream(@"C:\Data\pdfNew.pdf", FileMode.Create, FileAccess.ReadWrite);
            destFileStream.Write(fileBytes, 0, fileBytes.Length);
            destFileStream.Flush();
            destFileStream.Close();
        //https://stackoverflow.com/questions/4124106/add-a-page-to-pdf-document-using-itextsharp
        private static byte[] CopyPagesFromPDF(Stream sourcePDFFileStream, List<int> pageNumbers)
            if (pageNumbers == null || !pageNumbers.Any())
                throw new ArgumentNullException("pageNumbers is null or empty");
            pageNumbers.Sort();
            if (pageNumbers.First() < 1)
                throw new ArgumentException("the minum pageNumbers can't less than 0.");
            // we create a reader for the document
            PdfReader reader = new PdfReader(sourcePDFFileStream);
            if (pageNumbers.Last() > reader.NumberOfPages)
                throw new ArgumentException("the maxium pageNumbers can't great than max pageNumber");
            MemoryStream memoryStream = new MemoryStream();
            //Step 1: Create a Docuement-Object
            Document document = new Document();
                //Step 2: we create a writer that listens to the document
                PdfWriter writer = PdfWriter.GetInstance(document, memoryStream);
                //Step 3: Open the document
                document.Open();
                PdfContentByte cb = writer.DirectContent;
                int currPageNumber = 1;
                foreach (var pageNumber in pageNumbers)
                    document.SetPageSize(reader.GetPageSizeWithRotation(pageNumber));
                    document.NewPage();
                    document.PageCount = currPageNumber++;
                    PdfImportedPage page = writer.GetImportedPage(reader, pageNumber);
                    int rotation = reader.GetPageRotation(pageNumber);
                    if (rotation == 90 || rotation == 270)
                        cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(pageNumber).Height);
                        cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
                writer.Close();
            catch (Exception e)
                throw new Exception(e.Message);
            finally
                document.Close();
            return memoryStream.ToArray();