Thank you for additional information. There is Primary header in your document. As you may know HTML format is one page format and it does not support Headers/Footers natively. Currently Primary Header/Footer are exported to HTML. It is the reason why additional characters appear in the output HTML. You can use the following code to clear Headers:
// Open document.
Document doc = new Document("1.doc");
doc.Sections[0].HeadersFooters.Clear();
doc.Save("out.htm");