The NNPDF3.1 parton distribution function (PDF) set is an updated version of the NNPDF3.0 set, which was the first global PDF set validated by a closure test. The update incorporates recent data and methodological improvements, increasing the number of independent PDFs from seven to eight by including the charm PDF alongside light quarks and gluons. The new data include the D0 electron and muon W asymmetries, complete LHCb W and Z production measurements, and new ATLAS and CMS data on jet and electroweak boson production. The study also includes top-quark pair differential distributions and Z boson transverse momentum data from ATLAS and CMS. The impact of parametrizing charm is investigated, showing improved accuracy and stability of the PDFs. The new data significantly reduce uncertainties, with the most notable changes in light-quark flavor separation and gluon precision. The NNPDF3.1 PDFs are delivered as Hessian sets and optimized Monte Carlo sets with compressed replicas. The results are compared with recent LHC measurements at 13 TeV, and predictions for Higgs production cross-sections are provided. The strangeness and charm content of the proton are discussed in light of the improved dataset and methodology. The NNPDF3.1 PDFs are presented at LO, NLO, and NNLO accuracy, with all NLO and NNLO PDFs delivered as Hessian sets and Monte Carlo replicas. The results are validated and compared with other PDF sets, showing good agreement at the one-sigma level. The study highlights the importance of precise PDFs for LHC phenomenology and new physics searches.The NNPDF3.1 parton distribution function (PDF) set is an updated version of the NNPDF3.0 set, which was the first global PDF set validated by a closure test. The update incorporates recent data and methodological improvements, increasing the number of independent PDFs from seven to eight by including the charm PDF alongside light quarks and gluons. The new data include the D0 electron and muon W asymmetries, complete LHCb W and Z production measurements, and new ATLAS and CMS data on jet and electroweak boson production. The study also includes top-quark pair differential distributions and Z boson transverse momentum data from ATLAS and CMS. The impact of parametrizing charm is investigated, showing improved accuracy and stability of the PDFs. The new data significantly reduce uncertainties, with the most notable changes in light-quark flavor separation and gluon precision. The NNPDF3.1 PDFs are delivered as Hessian sets and optimized Monte Carlo sets with compressed replicas. The results are compared with recent LHC measurements at 13 TeV, and predictions for Higgs production cross-sections are provided. The strangeness and charm content of the proton are discussed in light of the improved dataset and methodology. The NNPDF3.1 PDFs are presented at LO, NLO, and NNLO accuracy, with all NLO and NNLO PDFs delivered as Hessian sets and Monte Carlo replicas. The results are validated and compared with other PDF sets, showing good agreement at the one-sigma level. The study highlights the importance of precise PDFs for LHC phenomenology and new physics searches.