Lately, Passive Intelligent Surfaces (PIS) are being recognized to play an important role in meeting the timely demand of low-cost green sustainable Internet of Things (IoT). In this paper, we focus on maximizing the sum received power among the energy harvesting IoT users by jointly optimizing the active precoder for multi-antenna power beacon and the passive constant-envelope precoding based phase shifters (PS) design for PIS. Here, a multiuser channel estimation protocol is first introduced to obtain the least-squares estimators for the underlying effective cascaded channel links involved in the PIS assisted multi-antenna wireless power transfer as desired for the optimal precoder and PS designing. Thereafter, new semi-closed-form expressions for the proposed optimal active and passive beamforming design are derived so as to meet the low-complexity requirements of IoT communications. Finally, the numerical results are presented to validate the key nontrivial analytical claims and demonstrate the significant performance enhancement in terms of sum harvested energy among IoT users over conventional designs.