pnmtoddif: 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/pnmtoddif - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmtoddif - from package: netpbm

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

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