  We present far-infrared spectroscopic observations, taken with the
Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space
Observatory, of the protoplanetary disk around the pre-main-sequence star HD
100546. These observations are the first within the DIGIT Herschel key program,
which aims to follow the evolution of dust, ice, and gas from young stellar
objects still embedded in their parental molecular cloud core, through the
final pre-main-sequence phases when the circumstellar disks are dissipated.
  Our aim is to improve the constraints on temperature and chemical composition
of the crystalline olivines in the disk of HD 100546 and to give an inventory
of the gas lines present in its far-infrared spectrum. The 69 \mu\m feature is
analyzed in terms of position and shape to derive the dust temperature and
composition. Furthermore, we detected 32 emission lines from five gaseous
species and measured their line fluxes. The 69 \mu\m emission comes either from
dust grains with ~70 K at radii larger than 50 AU, as suggested by blackbody
fitting, or it arises from ~200 K dust at ~13 AU, close to the midplane, as
supported by radiative transfer models. We also conclude that the forsterite
crystals have few defects and contain at most a few percent iron by mass.
Forbidden line emission from [CII] at 157 \mu\m and [OI] at 63 and 145 \mu\m,
most likely due to photodissociation by stellar photons, is detected.
Furthermore, five H2O and several OH lines are detected. We also found high-J
rotational transition lines of CO, with rotational temperatures of ~300 K for
the transitions up to J=22-21 and T~800 K for higher transitions.
