pdfcrop: 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/pdfcrop - from package: texlive-utils

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

On Debian possible default full paths are:

/usr/bin/pdfcrop - from package: texlive-extra-utils

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

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