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With data on the depth of maximum Xmax collected during more than a decade of operation of the Pierre Auger Observatory, we report on the inferences on the mass composition of UHECRs in the energy range E = 1017.2 – 1019.6 eV and on the measurements of the proton-air cross section for energies up to 1018.5 eV. We also present the results on Xmax obtained using the information on the particle arrival times recorded by the SD stations allowing us to extend the Xmax measurements up to 1020 eV. The inferences on mass composition, in particular using the data of the SD, are subject to systematic uncertainties due to uncertainties in the description of hadronic interactions at ultra-high energies. We discuss this problem with respect to the properties of the muonic component of extensive air-showers as derived from the SD data.