I do believe that Goodnotes does some of the best (if not the best) work at editing PDF powerpoint slides. One feature that I love about another competitor app however, is the ability to edit on the side the PDF slide aka slide notes. This would be a great way to add additional information (definitions, questions, images,stickers) and avoiding layering on an already cluttered slide. I think this would pair well with Goodnotes 'presentation' mode which I use quite often and pull up my own notes to present lecture slides.