pdf_callbacks: 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/pdf_callbacks - from package: rubygem-pdf-reader
/usr/lib/ruby/gems/1.8/gems/pdf-reader-1.1.1/bin/pdf_callbacks - from package: rubygem-pdf-reader

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

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