Following code will help you saving the workbook to an HttpResponse:
Workbook workbook = new Workbook();
//... Your code goes here
//Save in pdf format and send the file to the client browser, he may open the file in
//some application or save it to some location
workbook.Save(this.Response, "Report.pdf", ContentDisposition.Attachment, new XlsSaveOptions(SaveFormat.Pdf));
Moreover please make sure that your project is referencing the correct version of Aspose.Cells.dll. Because, we have tested saving HttpResponse. It is working fine. In case, if it is not please forward us your project so that we can further test it.
If you are building a web application, please use the Aspose.Cells.dll in the Net2.0 directory.
The dll in net3.5_ClientProfile directory is used only for the console application with Net frame client profile. Please check your project, it is possible that your project is referencing this dll. For complete reference, please read the readme.txt file in the bin directory.