Comment 3 for bug 362689

This is probably because cups-pdf was dropped from e.g. ubuntu-desktop dependencies, now that both GTK2 and QT (the graphic toolkits libraries used by GNOME and KDE, respectively) offer built-in printing management dialogs and the ability to produce PDF from there by design, so whatever APT backend 'update-manager' uses decided to automatically remove the package. My best guess is thus that this bug really should be assigned to update-manager.