diffpdf: 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/diffpdf - from package: diffpdf

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

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