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$HOOK_DISPLAY_PDF is not hook !!

It's a smarty variable.
You can find it in ./classes/pdf/HTMLTemplate.php

Find it:

/classes/pdf/HTMLTemplate.php

/pdf/delivery-slip.tpl

/pdf/invoice-b2b.tpl

/pdf/invoice.tpl

/pdf/order-return.tpl

/pdf/order-slip.tpl