If you know the start and end position of the text you want, you can simply get at it using the Mid function, which has the general form:
Mid(start, length)
For example:
MsgBox ActiveDocument.Text.Mid(0, 10) ' returns the first 10 characters of the document
i = ActiveDocument.Text.Count
MsgBox ActiveDocument.Text.Mid(0, i) ' returns all the characters of the document
MsgBox ActiveDocument.Text.Mid(10, 20) ' returns 20 characters starting at position 10