Is there a password to encrypt your document? If so, you can try to use the following code:
Document document = new Document();
document.LoadFromFile(@"E:\work\Documents\welcome.docx", FileFormat.Docx, "123456");
And I use a .NET Word component which is introduced here: http://www.codeproject.com/Catalogs/3773/Spire-Doc-for-NET.aspx
If no password or you don't know the password,I think it is a little complicated to decrypt.