  We present a detailed study of the helicity-dependent and
helicity-independent collinear parton distribution functions (PDFs) of the
nucleon, using the quasi-PDF approach. The lattice QCD computation is performed
employing twisted mass fermions with a physical value of the light quark mass.
We give a systematic and in-depth account of the salient features entering in
the evaluation of quasi-PDFs and their relation to the light-cone PDFs. In
particular, we give details for the computation of the matrix elements,
including the study of the various sources of systematic uncertainties, such as
excited states contamination. In addition, we discuss the non-perturbative
renormalization scheme used here and its systematics, effects of truncating the
Fourier transform and different matching prescriptions.
