gtkdoc-mkpdf: 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 Debian possible default full paths are:

/usr/bin/gtkdoc-mkpdf - from package: gtk-doc-tools

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

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