comparepdf: 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/comparepdf - from package: comparepdf

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

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