Adding and saving annotations/notes/highlights, fast full text search (aggressive preload), double page/single page views, form filling and saving (perhaps not even to the original PDF) are the minimum you really need for "working" with PDF...just "viewing" them is not really enough for serious academic work/research on PDF files. The more well integrated, the better. The functionality should look like in Acrobat, Okular, Mendeley, or similar tools.