Attachment
expects an absolute path.
You can convert your virtual path to an absolute path with
var absolutePath = Server.MapPath("~/Attachments/worddocument.doc")
And attach it with
Attachments.Add(new Attachment(absolutePath, MediaTypeNames.Application.Octet));
If you want to check if the file in the virtual directory exists, use
if (File.Exists(absolutePath))
...