  The purely gravitational evidence supporting the need for dark matter (DM)
particles is compelling and based on Galactic to cosmological scale
observations. Thus far, the promising weakly interacting massive particles
scenarios have eluded detection, motivating alternative models for DM. We
consider the scenarios involving the superheavy dark matter (SHDM) that
potentially can be emitted by primordial black holes (PBHs) and can decay or
annihilate into ultrahigh-energy (UHE) neutrinos and photons. The observation
of a population of photons with energies $E\ge 10^{11}$ GeV would imply the
existence of completely new physical phenomena, or shed some light on DM
models. Only the ultra-high energy cosmic ray observatories have the
capabilities to detect such UHE decay products via the measurements of UHE
photon induced extensive air showers. Using the upper bound on the flux of UHE
cosmic rays beyond $10^{11.3}$ GeV implying $J(>10^{11.3}~{\rm{GeV}})<
3.6\times 10^{-5}$ km$^{-2}$sr$^{-1}$y$^{-1}$, at the $90\%$ C.L. reported by
the Pierre Auger Observatory, we obtain global limits on the lifetime of the DM
particles with masses $10^{15}\le M_{X} \le 10^{17}$ GeV. The constraints
derived here are new and cover a region of the parameter space not yet
explored. We compare our results with the projected constraints from future
POEMMA and JEM-EUSO experiments, in order to quantify the improvement that will
be obtained by these missions.
  Moreover, assuming that an epoch of early PBHs domination introduces a unique
spectral break, $f_{\ast}$, in the gravitational wave spectrum, the frequency
of which is related to the SHDM mass, we map potential probes and limits of the
DM particles masses on the $f_{\ast}-M_{X}$ parameter space.
