pdfopt: 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/pdfopt - from package: ghostscript

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

On Debian possible default full paths are:

/usr/bin/pdfopt - from package: ghostscript

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

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