In this paper, we propose an online document delivery system which enables the sender to claim that documents are certainly delivered to the receiver. It is not difficult to realize this property by using a Trusted Third Party (TTP), but we focus on systems without TTP, considering a practical use. Our assumption is only the Public Key Infrastructure (PKI). Our work is based on the work by Shimizu et al., in which they applied the so-called ''gradual secret exchange protocol'' and implemented an e-mail exchange system using this protocol. The aim of this paper is to improve their system. We first point out that although the gradual secret exchange protocol is theoretically excellent, it is over-spec for our purpose. Hence we avoid using it and propose a simplified protocol, which has low computational cost. Using this protocol, we design a certified document delivery system based on the Half Agent Model proposed by Shimizu et al. Finally, we implement our protocol as a prototype system, and evaluate its performance. Experimental results show that our system runs efficiently with relatively small example documents, such as ones of size less than 30MB. © 2011 IEEE.