Editing is a loaded term. For some people, editing a PDF means changing a few words or a swapping out an old image for a new one, while for others it means altering metadata such as bookmarks, and for still others it means manipulating page order or adjusting print resolution. The authoritative answer nobody ever wants is: don't edit PDFs, edit the source and then export a new PDF. That's not always possible, though, and luckily there are some great tools to make all manner of edits possible.