The Pierre Auger Observatory is the most sensitive detector to primary photons
with energies above $\sim 0.2$ EeV.
It measures extensive air showers using a hybrid technique that combines a fluorescence detector
(FD) with a ground array of particle detectors (SD).
The signatures of a photon-induced air shower are a larger atmospheric depth at the shower maximum
($X_{max}$) and a steeper lateral distribution function, along with a lower number of muons with
respect to the bulk of hadron-induced background.
Using observables measured by the FD and SD, three photon searches in different energy bands are
performed.
In particular, between threshold energies of 1-10 EeV, a new analysis
technique has been developed
by combining the FD-based measurement of $X_{max}$ with the SD signal through a parameter
related to its muon content, derived from the universality of the
air showers.
This technique has led to a better photon/hadron separation and, consequently, to a higher
search sensitivity, resulting in a tighter upper limit than before.
The outcome of this new analysis is presented here, along with previous results
in the energy ranges
below 1 EeV and above 10 EeV.
From the data collected by the Pierre Auger Observatory in about 15 years of operation,
the most stringent constraints on the fraction of photons in the cosmic flux are set over almost
three decades in energy.