Thinking about it, two things do occur:
1) Why are you using a hardcoded file path:
Dim filePath As String = HttpContext.Current.Server.MapPath("../Doc_Uploads/Waybill.pdf")
Web apps and inherently multiuser: using a hardcoded file path will mean that all users will overwrite the same file.
2) That path is probably your problem: websites do not run under your user ID, the run under a special ID for IIS which has (deliberately) very restricted access to the rest of the system - so unless that path is fully opened for read and write permissions to pretty much all users the write attempt will fail in production.