If its a PDF and a functional/work/manual type of document (and not in DevonTHINK which is a whole other deal) it’s in a folder imaginatively called PDFs, stored on Dropbox and read by PDF Expert, my favorite PDF reader/editor.
If its a PDF and its a book or a magazine and not ‘useful’ or work-related they all live in Books, the Mac will launch it into Preview, the iPad or iPhone will read it directly in Books.
My son uses Papers for his research and academic cataloguing of his PDFs and Ive checked it out but documents such as that are all handled inside DevonTHINK for me.