private MemoryStream GetNewPdfByPageNum(PdfReader pdfReader, int pageNum)

{

    MemoryStream memoryStream = new MemoryStream();

    try

    {

        iTextSharp.text.Document document = new iTextSharp.text.Document();

        PdfCopy copy = new PdfCopy(document, memoryStream) { CloseStream = false };

        document.Open();

        copy.AddPage(copy.GetImportedPage(pdfReader, pageNum));

        document.Close();

    }

    catch { }

    return memoryStream;

}

var desktopPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);

var pdfBytes = new WebClient().DownloadData("http://....pdf");

var pdfReader = new PdfReader(pdfBytes);

for (int i = ; i < pdfReader.NumberOfPages + ; i++)

{

    var memoryStream = GetNewPdfByPageNum(pdfReader, i);

    FileStream file = new FileStream($"{desktopPath}/page{i}.pdf", FileMode.CreateNew);

    memoryStream.WriteTo(file);

    file.Dispose();

}

pdfReader.Close();