.TextEncoding - для сохранения. Для открытия:
Documents.Open(FileName, ConfirmConversions, ReadOnly, AddToRecentFiles, PasswordDocument, PasswordTemplate, Revert, WritePasswordDocument, WritePasswordTemplate, Format, Encoding, Visible, OpenConflictDocument, OpenAndRepair , DocumentDirection, NoEncodingDialog)

Encoding - Optional Variant. The document encoding (code page or character set) to be used by Microsoft Word when you view the saved document. Can be any valid MsoEncoding constant. For the list of valid MsoEncoding constants, see the Object Browser in the Visual Basic Editor. The default value is the system code page.

NoEncodingDialog - Optional Variant. True to skip displaying the Encoding dialog box that Word displays if the text encoding cannot be recognized. The default value is False.

т.е. примерно так:

Похоже, формат должен быть не текст (4), а wdOpenFormatEncodedText, т.е. 5