主题：把指定文件中的所有word文档转化为pdf

我有个文件夹，里面全部为word文档，想全部转化为pdf，代码如下，报错是什么原因？

Dim dlg As New FolderBrowserDialog
If dlg.ShowDialog = DialogResult.Ok Then
    For Each File As String In FileSys.GetFiles(dlg.SelectedPath)
        Dim rs As String = File.SubString(0,File.Length-4)
        Dim tg As String = rs & ".pdf"
        Dim app As New MSWord.Application
        try
            Dim doc = app.Documents.Open(File)
            doc.ExportAsFixedFormat(tg, MSWord.WdExportFormat.wdExportFormatPDF)
            doc.saved = True
            app.Quit
        catch ex As exception
            msgbox(ex.message)
            app.Quit
        End try
    Next
End If