  The determination of the parton distribution functions (PDFs) of the proton
will be an essential input for the physics program of a future 100 TeV hadron
collider. The unprecedented center-of-mass energy will require knowledge of
PDFs in currently unexplored kinematical regions such as the ultra low-x region
or the region of multi-TeV momentum transfers. In this contribution we briefly
summarise the studies presented in the PDF section of the upcoming report on
"Physics at a 100 TeV pp collider: Standard Model processes". First we map the
PDF kinematical coverage in the $(x,Q^2)$ plane, quantify PDF uncertainties,
and compute ratios of PDF luminosities between 100 TeV and 14 TeV. Then we show
how the extreme kinematics of such collider lead to a number of remarkable
PDF-related phenomena such as the top quark as a massless parton, an increased
role of photon-initiated processes and the possible need of PDFs with
high-energy resummation.
