You can also make a PageEditor enabled version like this and it should automatically take the Class attribute into account:
@Editable(Model, x => x.Link)
Or when you use RenderLink, you can pass a collection with the class attribute:
@RenderLink(x => x.Link, new System.Collections.Specialized.NameValueCollection { { "class", "btn btn-primary" } })
EDIT: Modified example to working code and added formatting example for Editable
You can specify a format for Editable:
@(Editable<YourModelType>(Model, x => x.Link, string.Format("<a href=\"{0}\" class=\"btn btn-primary\">{1}</a>", x.Link.Url, x.Link.Text)))