xpdf: 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/xpdf - from package: xpdf

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

On Debian possible default full paths are:

/usr/bin/xpdf - from package: xpdf

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

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