pdfopen: 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/pdfopen - from package: texlive-binaries

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

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