Whilst testing conversion to N-Up-PDF I noticed that it is usually possible to read 3x3 pages or 4x4 continuously, However I still think the conventional view of 2 pages at a time is much more intuitive. I normally only have a single page width so as not to lose focus The main use for N-Up is to minimise carbon footprint in handouts, thus best reserved for printing.
Anyway the suggestion is IF you need multiple page widths such as 3 or 4 at a time then my N-Up-PDF addin can be run whilst viewing a PDF and you can delete that converted PDF if you wish after reading it. Most easily by using the companion Delete Later Addin.
Note as a freebie (and intended for printing) N-up does not always adjust bookmarks or hyperlinks (the 2-up version tries to) so on occasion may discard most of those in the viewing copy. (The original file will still be kept)