Spaces are not barred in filenames and the code you posted (essentially as below) works without any problem with your filename example
Code:
Sub Macro1()
Dim retval As Object
Dim strName As String
On Error Resume Next
strName = Replace(ActiveDocument.FullName, ".docx", ".txt")
ActiveDocument.SaveAs2 FileName:=strName, FileFormat:=wdFormatText
ActiveDocument.Close 0
retval = Shell("notepad.exe " & strName, 1)
End Sub