Action TCPDF : customize TITLE with field value

Hi,
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I have successfully customize the file name of the generated pdf, thanks to faq : https://www.chronoengine.com/forums/posts/t100266/include-field-value-in-tcpdf-file-name
resulting this:
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<?php$form->data['pdf_file_name'] = $form->data['firstname'].'_'.$form->data['lastname'].'_whateverelse';?>

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But I cannot do the same with the TITLE of the document.
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I've tried to put a custom code before the action with this code:
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<?php$form->data['pdf_file_title'] = $form->data['firstname'].'_'.$form->data['lastname'].'_whateverelse';?>

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But it is not working.
In the same way I've tried to customize the SUBJECT. But still no luck.
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<?php$form->data['pdf_file_subject'] = $form->data['firstname'].'_'.$form->data['lastname'].'_whateverelse';?>

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Any advice?
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Thanks!