关键之处是，只有用户点击你的网页按钮，才可以执行下面的代码，下载文件。

Private Sub LinkButton1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles LinkButton1.Click
    downloadfile("document.pdf") '

End Sub


'This is the function that you have to use

Private Function downloadfile(ByVal strFile As String)
    Dim fs As FileStream
    Dim strContentType As String
    ' This is the important part, because you going to use the local path 

    'to get the file

    Dim strPath = Me.Server.MapPath(System.Configuration.ConfigurationSettings.AppSettings("uploadDirectory")) & "\"
    Dim strFileName As String = strFile
    fs = File.Open(strPath & strFileName, FileMode.Open)
    Dim bytBytes(fs.Length) As Byte
    fs.Read(bytBytes, 0, fs.Length)
    fs.Close()
    Response.AddHeader("Content-disposition","attachment; filename=" & strFileName)
    Response.ContentType = "application/octet-stream"
    Response.BinaryWrite(bytBytes)
    Response.End()
    Return True
End Function