pdfnup: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/pdfnup - from package: pdfjam

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/pdfnup - from package: texlive-extra-utils

You may install the required package with command apt-get install {package}

Error in other languages:
pdfnup: bevel nie gevind nie
pdfnup: comandă negăsită
pdfnup: comando não encontrado
pdfnup: commande introuvable
pdfnup: command not found
pdfnup: käsku ei ole
pdfnup: không tìm thấy lệnh
pdfnup: komanda nerasta
pdfnup: Kommando nicht gefunden.
pdfnup: kommandot finns inte
pdfnup: komut yok
pdfnup: nie znaleziono polecenia
pdfnup: níor aimsíodh an t-ordú
pdfnup: no se encontró la orden
pdfnup: no s'ha trobat l'ordre
pdfnup: opdracht niet gevonden
pdfnup: parancs nem található
pdfnup: perintah tidak ditemukan
pdfnup: príkaz nenájdený
pdfnup: příkaz nenalezen
pdfnup: команда не найдена
pdfnup: командата не е открита
pdfnup: 命令找不到
pdfnup: コマンドが見つかりません