Options.DefaultTextEncoding property

Returns or sets a MsoEncoding constant representing the code page, or character set, that Microsoft Word uses for all documents saved as encoded text files.

Namespace:  Microsoft.Office.Interop.Word
Assembly:  Microsoft.Office.Interop.Word (in Microsoft.Office.Interop.Word.dll)

MsoEncoding DefaultTextEncoding { get; set; }

Use the TextEncoding property to set the encoding for an individual document. To set encoding for HTML documents, use the Encoding property.