The filter for spin echo PD is linear: S = PD. The higher the proton density the more signal from "PD" weighting. The tissue filter is rather boring and there is not really anything to change.

The more protons per voxel the more signal the voxel will have, and the filter shows that this is a linear relationship. This makes sense, more protons per voxel results in more aligned spins and therefore more signal. As you will see on subsequent modules, you can create a PD weighted imaging by setting TR and TE such that there is no T1 or T2 weighting (i.e. there is no dependence of signal on tissue T1 or T2 values). All that is left is PD weighting. In general, every sequence has proton density weighting plus more or less T1 and T2 weightings depending on the values of TR and TE.