对 PDF 文档进行切片

如何在 C# 中对 PDF 文档进行切片.. 这是我用于切片的代码，但在切片后切片 10 MB 的 PDF 文档时，总大小为 20 MB 如何在切片时减少切片文档???请给出一个好的切片和压缩方法

public int ExtractPages(string sourcePdfPath)
    {
        int p = 0;
        try
        {
            iTextSharp.text.Document document;
            iTextSharp.text.pdf.PdfReader reader = new        iTextSharp.text.pdf.PdfReader(new iTextSharp.text.pdf.RandomAccessFileOrArray(sourcePdfPath), new ASCIIEncoding().GetBytes(""));
            if (!Directory.Exists(sourcePdfPath.ToLower().Replace(".pdf", "")))
            {
                Directory.CreateDirectory(sourcePdfPath.ToLower().Replace(".pdf", ""));
            }
            else
            {
                Directory.Delete(sourcePdfPath.ToLower().Replace(".pdf", ""), true);
                Directory.CreateDirectory(sourcePdfPath.ToLower().Replace(".pdf", ""));
            }
            for (p = 1; p <= reader.NumberOfPages; p++)
            {
                using (MemoryStream memoryStream = new MemoryStream())
                {
                    document = new iTextSharp.text.Document();
                    iTextSharp.text.pdf.PdfWriter writer = iTextSharp.text.pdf.PdfWriter.GetInstance(document, memoryStream);
                    writer.SetPdfVersion(iTextSharp.text.pdf.PdfWriter.PDF_VERSION_1_2);
                    writer.CompressionLevel = iTextSharp.text.pdf.PdfStream.BEST_COMPRESSION;
                    writer.SetFullCompression();
                    document.SetPageSize(reader.GetPageSize(p));
                    document.NewPage();
                    document.Open();
                    document.AddDocListener(writer);
                    iTextSharp.text.pdf.PdfContentByte cb = writer.DirectContent;
                    iTextSharp.text.pdf.PdfImportedPage pageImport = writer.GetImportedPage(reader, p);
                    int rot = reader.GetPageRotation(p);
                    if (rot == 90 || rot == 270)
                    {
                        cb.AddTemplate(pageImport, 0, -1.0F, 1.0F, 0, 0, reader.GetPageSizeWithRotation(p).Height);
                    }
                    else
                    {
                        cb.AddTemplate(pageImport, 1.0F, 0, 0, 1.0F, 0, 0);
                    }
                    document.Close();
                    document.Dispose();
                    File.WriteAllBytes(sourcePdfPath.ToLower().Replace(".pdf", "") + "/" + p + ".pdf", memoryStream.ToArray());
                }
            }
            reader.Close();
            reader.Dispose();
        }
        catch
        {
        }
        finally
        {
            GC.Collect();
        }
        return p - 1;