Try specifying the content type of the attachment:
Attachment pdfAttachment = new Attachment(new MemoryStream(bytes), "CC" + result + ".pdf");
pdfAttachment.ContentType.MediaType = System.Net.Mime.MediaTypeNames.Application.Pdf;
Msg.Attachments.Add(pdfAttachment);
EDIT:
Based on the discussion in the comments below, the problem was an un-closed
FileStream
in the mail sending code, resulting in a "file in use" error.
The solution was to wrap the
FileStream
in a
using
block so that it was always closed.