pdfjoin: 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/pdfjoin - from package: pdfjam

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

On Debian possible default full paths are:

/usr/bin/pdfjoin - from package: texlive-extra-utils

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

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