  We illustrate how electron Parton Distribution Functions (PDFs) with
next-to-leading collinear logarithmic accuracy must be employed in the context
of perturbative predictions for high-energy $e^+e^-$-collision processes. In
particular, we discuss how the renormalisation group equation evolution of such
PDFs is affected by the presence of multiple fermion families and their
respective mass thresholds, and by the dependences on the choices of the
factorisation and renormalisation schemes. We study the impact of the
uncertainties associated with the PDFs on physical cross sections, in order to
arrive at realistic precision estimates for observables computed with
collinear-factorisation formulae. We do so by presenting results for the
production of a heavy neutral object as well as for $t\bar{t}$ and $W^+W^-$
pairs, including next-to-leading-order effects of electroweak origin.
