Answer
Compare the actual underlying data rows in the report's own _REP view for both runs, rather than just comparing the two final rendered PDF outputs — there will be multiple rows of data per report result key/job in that view. Differences visible directly in the _REP view's row-level data for each specific result key are much easier to pin down than trying to spot differences purely by eyeballing two PDF outputs, which adds an extra layer of potentially misleading formatting/rendering noise on top of whatever the actual underlying data difference turns out to be.