okular: 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/okular - from package: kdegraphics

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

On Debian possible default full paths are:

/usr/bin/okular - from package: okular

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

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