qpdf: 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/qpdf - from package: qpdf

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

On Debian possible default full paths are:

/usr/bin/qpdf - from package: qpdf

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

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