No, it is neither legal nor  proper for the secretary to do this. The landlord and his/her/its staff may not enter your rental premises without your permission or consent simply to leave paper or documents--including invoices or demands for past due rent, or even legal/court documents, like a summons or complaint--in your home. She could have legally slid it under your front door (if there was  sufficient room to do so), so long as she did not unlock your door or otherwise enter the premises.