#!/bin/bash

mkdir -p bak
for i in *.pdf; do
  if a=`expr "$i" : '[0-9]*_[0-9]*_\([0-9]*\).*'`; then
    if [ -e $a.pdf -a $a.pdf -nt $i ]; then
      echo "Append $a.pdf to $i"
      mv $i bak
      pdftk bak/$i $a.pdf cat output $i
      mv $a.pdf bak
    fi
  fi
done