PDFs of G28.37+0.07 and its associated GMC, derived from the Herschel column density map. The upper panel shows the PDF of the IRDC, only from the pixels inside the ellipse shown in Fig. 2. The lower panel shows the PDF obtained for the whole GMC (all pixels inside the contour indicated in Fig. 3, right). The error bars were calculated using Poisson statistics. The left y-axis gives the normalised probability p(η) and the right y-axis indicates the number of pixels per log bin. The upper x-axis is the visual extinction and the lower x-axis is the logarithm of the normalised column density. The green curve shows a log-normal fit to the PDF from the GMC. The red line indicates a power-law fit to the high column density tail with the slope s together with its error and the reduced X² goodness-of-fit. The exponent α of an equivalent spherical density distribution ρ(r) ∝ r^{− α} is also indicated in the panel. The dashed line in the upper panel indicates the completeness level; the PDF left of this line is incomplete.