  We report the first hard X-ray observations with NuSTAR of the BL Lac type
blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and
gamma-ray data from the Fermi Large Area Telescope, obtained in April 2013 when
the source was in a very low flux state. A joint NuSTAR and XMM spectrum,
covering the energy range 0.5 - 60 keV, is best described by a model consisting
of a log-parabola component with curvature beta = 0.3(+0.2,-0.1) and a (local)
photon index 3.04 +/- 0.15 at photon energy of 2 keV, and a hard power-law tail
with photon index 2.2 +/- 0.4. The hard X-ray tail can be smoothly joined to
the quasi-simultaneous gamma-ray spectrum by a synchrotron self-Compton
component produced by an electron distribution with index p = 2.2. Assuming
that the power-law electron distribution extends down to the minimum electron
Lorentz factor gamma_min = 1 and that there is one proton per electron, an
unrealistically high total jet power L_p of roughly 10^47 erg/s is inferred.
This can be reduced by two orders of magnitude either by considering a
significant presence of electron-positron pairs with lepton-to-proton ratio of
at least 30, or by introducing an additional, low-energy break in the electron
energy distribution at the electron Lorentz factor gamma_br1 of roughly 100. In
either case, the jet composition is expected to be strongly matter-dominated.
