In this paper the problem is posed of the formulation of the so-called ab initio approach to the statistical description of the Boltzmann-Sinai N-body classical dynamical system (CDS) formed by identical smooth hard spheres. This amounts to introducing asuitably generalized version of the axioms of Classical Statistical Mechanics. The latter involve a proper definition of the functional setting for the N-body probability density function (PDF), so that it includes also the case of the deterministic N-body PDF. In connection with this issue, a further development concerns the introduction of modified collision boundary conditions which differ from the usual ones adopted in previous literature. Both features are proved to be consistent with the validityof exact H-theorems for the N-body and 1-body PDFs, respectively. Consequences of the axiomatic approach which concern the conditions of validity of the Boltzmann kinetic equation and the Boltzmann H-theorem are investigated. In particul