iText:

Document doc = new Document(PageSize.A4, 50, 50, 25, 25);

PdfWriter writer = PdfWriter.GetInstance(doc, ms);

PdfContentByte contentPlacer;

writer.SetPdfVersion(PdfWriter.PDF_VERSION_1_5);

writer.CompressionLevel = PdfStream.BEST_COMPRESSION;

string pdfFile = @"D:\Test.pdf";

PdfReader reader = new PdfReader(pdfFile);

PdfStamper stamper = new PdfStamper(reader, new FileStream(@"D:\Test1.pdf", FileMode.Create), PdfWriter.VERSION_1_5);

stamper.FormFlattening = true;

stamper.SetFullCompression();

stamper.Close();