Paul VI himself makes clear that Populorum Progressio (PP) is fundamentally a political document, particularly in para. 47.   PP is not meant simply  to be a treatise on personal piety, and cannot be analyzed or interpreted from that viewpoint, as many a traditional Catholic has attempted, at the expense of a diluted and distorted Gospel!  The late Peter Hebblethwaite's bio of Paul VI is a good backgrounder, and points out that PP flows logically from Paul VI's address at the UN in 1965 (politics on a global scale!).