pstopnm: command not found

Have You tried accessing the binary with the full path?

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/pstopnm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pstopnm - from package: netpbm

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