using (FileStream fileStream = File.OpenRead(filesToZip[i]))
{

  byte[] buffer = new byte[fileStream.Length];
  fileStream.Read(buffer, 0, buffer.Length);

  entry.Size = fileStream.Length;
  fileStream.Close();

  crc.Reset();
  crc.Update(buffer);
  entry.Crc = crc.Value;

   //add the zip entry to the stream.
   zipStream.PutNextEntry(entry);
   zipStream.Write(buffer, 0, buffer.Length);
}

e.UpdateProgress(0);
        this.memoryStream = new MemoryStream();
        ZipOutputStream zip = null;
        ArrayList filenames = new ArrayList();

        foreach (ListItem li in cblDrugs.Items)
        {
            if (li.Selected == true)
            {
                // Add to zip file
                filenames.Add(CommonLib.GlobalData.PDFDirectory + li.Value + "_DataSheet.pdf");
            }
        }

        string[] filesToZip = new string[filenames.Count];
        int z = 0;
        foreach (string sFilename in filenames)
        {
            filesToZip[z] = sFilename;
            z++;
        }

        ZipOutputStream zipStream = new ZipOutputStream(this.memoryStream);
        zipStream.SetLevel(9); // 0 - store only to 9 - means best compression

        Crc32 crc = new Crc32();

        int x = 0;
        int denom = (100 / filesToZip.Length);
        for (int i = 0; i < filesToZip.Length; i++)
        {
            // Must use a relative path here so that files show up in the Windows Zip File Viewer
            // .. hence the use of Path.GetFileName(...)
            string fileName = Path.GetFileName(filesToZip[i]);

            //each zipentry is one file in the zip archive
            ZipEntry entry = new ZipEntry(fileName);
            entry.DateTime = DateTime.Now;

            // Read in the files one by one.
            using (FileStream fileStream = File.OpenRead(filesToZip[i]))
            {
                byte[] buffer = new byte[fileStream.Length];
                fileStream.Read(buffer, 0, buffer.Length);

                entry.Size = fileStream.Length;
                fileStream.Close();

                crc.Reset();
                crc.Update(buffer);
                entry.Crc = crc.Value;

                //add the zip entry to the stream.
                zipStream.PutNextEntry(entry);
                zipStream.Write(buffer, 0, buffer.Length);
            }

            System.Threading.Thread.Sleep(300);
            x += denom;
            e.UpdateProgress(x);
            
        }

        zipStream.Finish();
        e.UpdateProgress(100, "Documents ready for download!");
            
        Response.Clear();
        Response.AddHeader("Content-Type", "binary/octet-stream");
        Response.AddHeader("Content-Length", zip.Length.ToString());

        //the filename might be dynamic as well.
        Response.AddHeader("Content-Disposition",
             "attachment; filename=DataSheets.zip; size="
             + zipStream.Length.ToString());
        Response.Flush();

        //one line of code to send back the stream.  Sweet!
        Response.BinaryWrite(this.memoryStream.ToArray());

        Response.Flush();
        Response.End();

        this.memoryStream.Close();
        this.memoryStream.Dispose();