Recently, several observatories have discovered photons of cosmic origin with maximum energies in the PeV ($10^{15}$ eV) range. Photons at these energies might be produced as by-products from particle acceleration in so-called PeVatrons, which are widely assumed to be the source of a large part of galactic cosmic rays. The first PeVatron to be firmly established was the Crab nebula, a pulsar wind nebula. Another PeVatron may be located in the Galactic Center region. However, the measurement of PeV photons from several other sources indicates the presence of further PeVatrons, which may also be associated with supernova remnants. In this contribution, we present a compilation of recent measurements of PeV photons by LHAASO and HAWC. We extrapolate the measured energy spectra up to the ultra-high-energy (UHE, $E>10^{17}$ eV) regime to obtain an estimate of the required sensitivity for the measurement of UHE photons from specific source candidates. One goal of this study is to evaluate the potential of present and future cosmic-ray observatories, for example the Pierre Auger Observatory, to detect such photons, which could provide complementary information on the sources of cosmic rays beyond the PeV regime—a key objective of current efforts in multimessenger astronomy.