pdftoppm: 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/pdftoppm - from package: poppler-utils

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

On Debian possible default full paths are:

/usr/bin/pdftoppm - from package: poppler-utils

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

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