Paper and paperboard are promising materials for a sustainable future, but expanding their applications requires robust simulation tools to predict material behavior accurately. This demands an extensive insight into the various inelastic phenomena, which characterize the material. Therefore, this work focuses on the theoretical derivation and numerical investigation of a comprehensive elasto-plastic material model for finite deformations. The model unifies the distinct in-plane and out-of-plane behavior of the material. This is reflected in the formulation of both the free energy and yield function. The latter one is based on a well-established formulation, which has been adapted to resolve inconsistencies related to initial yield stresses and plastic strain ratios. Furthermore, the model addresses non-isotropic hardening using coupled internal hardening variables. Lastly, to demonstrate its capabilities, the model has been applied to a number of single-element and structural numerical examples. The model's ability to predict the material response of paper and paperboard under complex mechanical loading has the potential to enhance simulations of advanced 3D forming processes, leading to an increasing utilization of the material in various industrial applications.