  PDFs can be studied directly using lattice QCD by evaluating matrix elements
of non-local operators. A number of groups are pursuing numerical calculations
and investigating possible systematic uncertainties. One systematic that has
received less attention is the effect of calculating in a finite spacetime
volume. Here we present first attempts to assess the role of the finite volume
for spatially non-local operators. We find that these matrix elements may
suffer from large finite-volume artifacts and more careful investigation is
needed.
