pdfcrack: 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/pdfcrack - from package: pdfcrack

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

On Debian possible default full paths are:

/usr/bin/pdfcrack - from package: pdfcrack

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

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