pdfunite: 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 Debian possible default full paths are:

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

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

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