Local resolution of a crystal structure in PDB File (protein data bank)
The question has false statements:


  Usually, the resolution of the PDB is assigned the resolution of the highest resolved part of the protein/nucleic acid.


Usually (in ~90% of the PDB entries) the experimental method used is X-ray crystallography. In such case the resolution refers to a distance between crystal lattice planes: it is assigned the value of the smallest distance between lattice planes that was resolved in the diffraction pattern.

Crystallographic experiment measures reflection. Each reflection corresponds to a set of lattice planes in the crystal. The spacing between these planes, d, is the measure of resolution. "Resolution range 3 - 20 Angstroms" means that the reflections used in the refinement had d between 3 and 20 Angstroms.

The resolution is simply dmin - the smallest distance between crystal lattice planes that resulted in a reflection usable in the refinement. Each reflection contributes to each point in the electron density map, so while we can talk about local quality of the structure, the resolution cannot be local.

Unless the data comes from a Cryo-EM experiment, where the resolution is defined differently and people can talk about local resolution.