  Aims: We investigate the composition of Titan's stratosphere from new
medium-resolution far-infrared observations performed with the full range of
Herschel's Photodetector Array Camera and Spectrometer (PACS) (51-220 $\mu$m at
a resolution $\lambda$/$\Delta \lambda$ ranging from 950 to 5500 depending on
wavelength and grating order). Methods: Using PACS, we obtained the spectral
emission of several features of the Titan's stratosphere. We used a
line-by-line radiative transfer code and the least-squares fitting technique to
infer the abundances of the trace constituents. Results: Numerous spectral
features attributable to CH$_4$, CO, HCN, and H$_2$O are present. From the flux
density spectrum measured and by a detailed comparison with synthetic spectra,
we constrain the stratospheric abundance of CH4, which is assumed to be
constant with altitude, to be 1.29 $\pm$ 0.03%. Similarly, we constrain the
abundance of CO to be 50 $\pm$ 2 ppm, and the HCN vertical distribution
consistent with an increase from 40 ppb at $\sim$100 km to 4 ppm at $\sim$200
km, which is an altitude region where the HCN signatures are sensitive.
Measurements of three H$_2$O rotational lines confirm the H$_2$O distribution
profile recently obtained with Herschel. Furthermore, we determine the isotopic
ratios $^{12}$C/$^{13}$C in CO and HCN to be 124 $\pm$ 58, and 66 $\pm$ 35,
respectively. Comparisons between our results and the values derived with other
instruments show that our results are consistent with the vertical
distributions and isotopic ratios in previous studies, except for the HCN
distribution obtained with Cassini/CIRS, which does not agree with the PACS
lines at the 1-sigma confidence interval.
