pcl_pcd2png: 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/pcl_pcd2png - from package: pcl-tools

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

On Debian possible default full paths are:

/usr/bin/pcl_pcd2png - from package: pcl-tools

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

Error in other languages:
pcl_pcd2png: bevel nie gevind nie
pcl_pcd2png: comandă negăsită
pcl_pcd2png: comando não encontrado
pcl_pcd2png: commande introuvable
pcl_pcd2png: command not found
pcl_pcd2png: käsku ei ole
pcl_pcd2png: không tìm thấy lệnh
pcl_pcd2png: komanda nerasta
pcl_pcd2png: Kommando nicht gefunden.
pcl_pcd2png: kommandot finns inte
pcl_pcd2png: komut yok
pcl_pcd2png: nie znaleziono polecenia
pcl_pcd2png: níor aimsíodh an t-ordú
pcl_pcd2png: no se encontró la orden
pcl_pcd2png: no s'ha trobat l'ordre
pcl_pcd2png: opdracht niet gevonden
pcl_pcd2png: parancs nem található
pcl_pcd2png: perintah tidak ditemukan
pcl_pcd2png: príkaz nenájdený
pcl_pcd2png: příkaz nenalezen
pcl_pcd2png: команда не найдена
pcl_pcd2png: командата не е открита
pcl_pcd2png: 命令找不到
pcl_pcd2png: コマンドが見つかりません