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[abridged] We present far-infrared spectroscopic observations of PMS starstaken with Herschel/PACS as part of the DIGIT key project. The sample includes22 Herbig AeBe and 8 T Tauri sources. Multiple atomic fine structure andmolecular lines are detected at the source position: [OI], [CII], CO, OH, H_2O,CH^+. The most common feature is the [OI] 63micron line detected in almost allof the sources followed by OH. In contrast with CO, OH is detected toward bothHerbig AeBe groups (flared and non-flared sources). An isothermal LTE slabmodel fit to the OH lines indicates column densities of 10^13 < N_OH < 10^16cm^-2, emitting radii 15 < r < 100 AU and excitation temperatures 100 < T_ex <400 K. The OH emission thus comes from a warm layer in the disk at intermediatestellar distances. Warm H_2O emission is detected through multiple lines towardthe T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBesystems HD 104237, HD 142527, HD 163296 (through line stacking). Overall,Herbig AeBe sources have higher OH/H_2O abundance ratios across the disk thando T Tauri disks, from near- to far-infrared wavelengths. Far-infrared CH^+emission is detected toward HD 100546 and HD 97048. The slab model suggestsmoderate excitation (T_ex ~ 100 K) and compact (r ~ 60 AU) emission in the caseof HD 100546. The [CII] emission is spatially extended in all sources where theline is detected. This suggests that not all [CII] emission is associated withthe disk and that there is a substantial contribution from diffuse materialaround the young stars. The flux ratios of the atomic fine structure lines areconsistent with a disk origin for the oxygen lines for most of the sources.

DIGIT survey of far-infrared lines from protoplanetary disks I