Thin films and interfaces of crystalline organic dyes with semiconducting properties attracted a lot of attention in the last decade due to their numerous applications in electronics and optoelectronics. One of the most studied molecules is 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA); an archetypal organic material that can grow into multilayer films. Despite the great interest and intensive investigations, its optical properties are still not completely understood. The interpretations range from the Wannier-Mott exciton model to models of excitons of small radii. In the present work, we apply the Frenkel exciton model in order to describe the optical behavior of the solid phase of PTCDA, influenced by the transfer of excitations between different molecules. We are able to model the anisotropy of dielectric tensor, lineshape of the complex index of refraction, exciton dispersion and the large Stokes shift between absorption and photoluminescence, results of electron-energy loss spectroscopy, and photoluminescence transition energies and decay times. In addition, we made an extension of the model towards ultrathin PTCDA films.