I have a different interpretation -- that the language is to avoid a context in which a tenant thinks that being served with a notice is a game of "tag", and either refuses to touch the papers that the landlord is trying to serve or won't open the door for service. I see nothing that would require a signature in order for personal service to be valid. However, under the language that notice "may be given by (a) personal delivery, or (b) certified mail, returned receipt requested", if the landlord doesn't have a return receipt and the tenant says that the notice never arrived, that provision of the lease would seem to rule out the landlord's argument that notice was properly given.

