xfig-pdf-viewer: 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/xfig-pdf-viewer - from package: xfig-doc

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

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