Not yet reviewed by Pith; the record is open.

Accurate Standard Model predictions of proton-proton collisions are essential for interpreting the current and forthcoming experimental measurements from high-energy colliders. The quest for physics beyond the Standard Model is in fact strongly related to a reliable determination of systematic uncertainties, among which the theoretical ones have nowadays become a significant component. In particular, an important role is played by the universal Parton Distribution Functions (PDFs). This thesis presents a collection of studies aimed at improving the accuracy and precision of collinear PDF determinations. By employing the NNPDF methodology, we present recent polarized and unpolarized PDF fits including higher order QCD corrections and theoretical uncertainties. Additionally, we examine the presence of non-vanishing intrinsic charm and its phenomenological implications.