pdfmerge: 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/pdfmerge - from package: pdfmerge

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

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