  This paper presents an analysis at next-to-next-to-leading order in the
theory of quantum chromodynamics for the determination of a new set of proton
parton distribution functions using diverse measurements in $pp$ collisions at
$\sqrt{s} = 7$, 8 and 13 TeV, performed by the ATLAS experiment at the Large
Hadron Collider, together with deep inelastic scattering data from $ep$
collisions at the HERA collider. The ATLAS data sets considered are
differential cross-section measurements of inclusive $W^{\pm}$ and $Z/\gamma^*$
boson production, $W^{\pm}$ and $Z$ boson production in association with jets,
$t\bar{t}$ production, inclusive jet production and direct photon production.
In the analysis, particular attention is paid to the correlation of systematic
uncertainties within and between the various ATLAS data sets and to the impact
of model, theoretical and parameterisation uncertainties. The resulting set of
parton distribution functions is called ATLASpdf21
