pdns_server: 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/sbin/pdns_server - from package: pdns

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

On Debian possible default full paths are:

/usr/sbin/pdns_server - from package: pdns-server

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

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