pdfconcat: 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/pdfconcat - from package: xpaint

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

On Debian possible default full paths are:

/usr/bin/pdfconcat - from package: xpaint

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

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