VB.Net: Cannot download stream file generated with ZipOutputStream - vb.net

Question

I'm trying to zip a set of pdf files and send to client as download.
No matter what combinations of Response settings I try, the code doesn't throw any exception and apparently the zip file stream is created fine, but the file is not sent to the client as a download and when you hit the download button nothing happens.
Private Sub lkbDownloadPdfs_Click(sender As Object, e As System.EventArgs) Handles aDownloadPdfs.ServerClick
Try
Dim WSStockToolAuthTokenUrl As String = ConfigurationManager.AppSettings("WSStockToolAuthTokenUrl")
Dim auth As AuthenticationHeader = Utility.GetAuthenticationForStockToolToken()
Dim token As String = Utility.GetInitializationToken(WSStockToolAuthTokenUrl, auth.UserName, auth.password)
Response.Clear()
Response.ContentType = "application/zip"
Response.AppendHeader("Content-Disposition", "attachment; filename=files.zip")
If (token IsNot Nothing) Then
Dim result As String = PDFApiCallResult(token)
Dim pdfPathList As List(Of String) = Utility.GeneratePDFList(result)
If (pdfPathList.Count = 1) Then
Dim pdfPath As String = pdfPathList.ElementAt(0)
Dim strFile As String
Dim strmZipOutputStream = New ZipOutputStream(Response.OutputStream)
strmZipOutputStream.SetLevel(9)
Dim objCrc32 As New Crc32()
For Each strFile In pdfPathList
Dim Client As WebClient = New WebClient()
Dim strmFile As Stream = Client.OpenRead(strFile)
Dim reader As StreamReader = New StreamReader(strmFile)
Dim Content As String = reader.ReadToEnd()
Dim abyBuffer(Convert.ToInt32(Content.Length - 1)) As Byte
Dim sFile As String = Path.GetFileName(strFile)
Dim theEntry As ZipEntry = New ZipEntry(sFile)
theEntry.DateTime = DateTime.Now
theEntry.Size = Content.Length
strmFile.Close()
objCrc32.Reset()
objCrc32.Update(abyBuffer)
theEntry.Crc = objCrc32.Value
strmZipOutputStream.PutNextEntry(theEntry)
strmZipOutputStream.Write(abyBuffer, 0, abyBuffer.Length)
Next
strmZipOutputStream.Flush()
strmZipOutputStream.Finish()
strmZipOutputStream.Close()
Response.Flush()
Response.Close()
Response.SuppressContent = True
HttpContext.Current.ApplicationInstance.CompleteRequest()
Else
End If
End If
Catch ex As Exception
ExceptionManager.Publish(ex)
End Try
End Sub
Any help? (If you have working C# code I could try to convert it to vb.net too)
Update 1: This is the aspx where the link which does the callback resides:
<asp:UpdatePanel runat="server" ID="updImages" UpdateMode="Conditional">
...
DOWNLOAD PDFS
...
<asp:AsyncPostBackTrigger ControlID="lkbAddToWishlist" />
<asp:AsyncPostBackTrigger ControlID="ddlCustomizations" />
</asp:UpdatePanel>
I've read that maybe the response is not working at all because of the ajax way the update panel does the postback, but not sure about that and how to deal with that.