You can not simply open RFT file as input.
following code is a simple example to save RTF to TXT file.
Code:
Public Sub Word2Text(wordFile As String, textFile As String)
'wordfile is the file name with path of input RTF file
'textFile is the file name with path for output
Dim wordApp As Object
Dim wordDoc As Object
Dim p As Long
Set wordApp = GetObject("", "word.Application")
' wordApp.Visible = True
Set wordDoc = wordApp.Documents.Open(wordFile)
Open textFile For Output As #1
For p = 1 To wordDoc.Range.paragraphs.Count
Print #1, CStr(wordDoc.Range.paragraphs(p))
Next
Close #1
wordDoc.Close
wordApp.Quit
Set wordApp = Nothing
Set wordDoc = Nothing
End Sub