~$ pdfnup --help
This is pdfnup version 1.20

Usage: pdfnup args
where args must include source pdf filename(s) and optionally also
* a specification, such as --nup 2x1 (two pages side by side)
--nup 1x2 (two pages stacked vertically)
--nup 2x2 etc
(in general mxn, where m and n are single-digit integers)
* a list or range of pages to be included, for example
--pages 3-6
--pages 2,8,4,5
--pages all
* a LaTeX papersize, for example --paper a4paper
--paper letterpaper
* the output page orientation, one of --orient landscape
--orient portrait
--orient auto
(auto guesses orientation; in the case of --nup 2x2,
--nup 3x3, etc., auto produces output pages the same size
as the logical pages being n-upped.)
* one of --frame true
--frame false
according to whether or not a thin line is required around pages
* a page-trimming specification such as
--trim "1cm 1cm 1cm 1cm"
(Note that trimming does not mix well with --frame true.)
* an "offset" specification such as
--offset "1cm 0.5cm"
to offset the position of output pages (see the pdfpages manual)
* a "delta" specification such as
--delta "1cm 1cm"
to put space between logical pages (see the pdfpages manual)
* a "scale" specification such as
--scale 0.91
to scale the output up or down in size (decrease or increase margins)
* one of --openright true
--openright false
according to whether or not a blank page should be inserted before
the first logical page
* one of --turn true
--turn false
according to whether or not landscape pages should be displayed in
landscape orientation
* one of --noautoscale true
--noautoscale false
for scaling of logical pages to fit (or not)
* one of --column true
--column false
according to whether or not column-major order should be used
* one of --columnstrict true
--columnstrict false
for whether or not the *last* page should be column-major ordered
regardless of how full it is
* a specific name for the output file, e.g. --outfile my2up.pdf
* one of --tidy true
--tidy false
according to whether or not temporary files should be deleted immediately.
If --tidy false is used, such files are left in /var/tmp.

Default arguments for you at this site are
--frame false --nup 2x1 --paper a4paper --orient auto --pages all --trim "0 0 0 0" --delta "0 0" --offset "0 0" --scale 1.0 --turn true --noautoscale false --openright false --column false --columnstrict false --tidy true

For information and version history see http://www.warwick.ac.uk/go/pdfjam