ppmtoacad: 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/ppmtoacad - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtoacad - from package: netpbm

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

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