How about renaming the file after it is created?
Code:
Private Sub cmdEmail_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdEmail.Click
        Dim ExportOptions As New CrystalDecisions.Shared.ExportOptions
        Dim EMailOptions As New CrystalDecisions.Shared.MicrosoftMailDestinationOptions
        Try
            ExportOptions.ExportFormatType = ExportFormatType.PortableDocFormat
            ExportOptions.ExportDestinationType = ExportDestinationType.MicrosoftMail
            'assuming it gets created with the above line.  If not, then move these
            'two lines as appropriate (and, of course, make sure the path
            'is correct
            Dim myFile As New System.IO.FileInfo("c:\untitled.txt")
            myFile.MoveTo("c:\report.pdf")
            EMailOptions.MailToList = txtEmail.Text
            EMailOptions.MailSubject = "Online Report"
            EMailOptions.MailMessage = "Report..."
            ExportOptions.ExportDestinationOptions = EMailOptions
            CrReport.Export(ExportOptions)
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
End Sub