  LOFAR offers the unique capability of observing pulsars across the 10-240 MHz
frequency range with a fractional bandwidth of roughly 50%. This spectral range
is well-suited for studying the frequency evolution of pulse profile morphology
caused by both intrinsic and extrinsic effects: such as changing emission
altitude in the pulsar magnetosphere or scatter broadening by the interstellar
medium, respectively. The magnitude of most of these effects increases rapidly
towards low frequencies. LOFAR can thus address a number of open questions
about the nature of radio pulsar emission and its propagation through the
interstellar medium. We present the average pulse profiles of 100 pulsars
observed in the two LOFAR frequency bands: High Band (120-167 MHz, 100
profiles) and Low Band (15-62 MHz, 26 profiles). We compare them with
Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations
at higher frequencies (350 and1400 MHz) in order to study the profile
evolution. The profiles are aligned in absolute phase by folding with a new set
of timing solutions from the Lovell Telescope, which we present along with
precise dispersion measures obtained with LOFAR. We find that the profile
evolution with decreasing radio frequency does not follow a specific trend but,
depending on the geometry of the pulsar, new components can enter into, or be
hidden from, view. Nonetheless, in general our observations confirm the
widening of pulsar profiles at low frequencies, as expected from
radius-to-frequency mapping or birefringence theories. We offer this catalog of
low-frequency pulsar profiles in a user friendly way via the EPN Database of
Pulsar Profiles (http://www.epta.eu.org/epndb/).
