#!/bin/bash
x=$(( $(identify odd.pdf | wc -l) - 1 ))
for i in `seq 0 $x`; do convert -density 300 odd.pdf[$i] $(($i * 2 + 1 )).pdf~; done
for i in `seq $x -1 0`; do convert -density 300 even.pdf[$(( $x - $i))] $((($i + 1) * 2 )).pdf~; done
eval convert -adjoin -density 300 {1..$((($x + 1) * 2))}.pdf~ complete.pdf && rm *.pdf~