Despite the gargantuan market potential, Printed Electronics-only (PE-only) circuits and systems on flexible substrate remain nascent. There are a number of reasons for this, including the inavailability of Process Development Kits (PDKs) to facilitate PE circuits and systems design and to predict manufacturability (due to the innately high process variations of PE). In this paper, we describe an "Open-Platform" PDK for our Fully-Additive Low-Temperature All-Air printing process with Very Low Process Variations. Our PDK embodies a novel simple yet accurate transistor model that can not only accurately model the printed transistors depending on their layout and when they are flat (unbent substrate) but also accurately model the process variations when they are bent (bent substrate). This PDK for bending is important for PE circuits and systems whose substrate is adhered to uneven surfaces or bent to fit odd spaces, thereby expanding the applicability of PE circuits and systems. The efficacy of the Open-Platform and the proposed PE transistor model is verified by means of comparisons between simulations and measurements of basic individual printed electronic elements and for several fundamental printed digital and analog circuits. These comparisons include when the substrate is flat and bent, depicting that bending is not only detrimental, but may conversely be advantageously exploited. The proposed PDK is compatible with commercial computer aided design simulation tools.