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I have a custom module with following configuration.
$config['modules']['applications'] = array(
    'preview_path' => 'applications',
    'table_actions' => array(
        'PDF' => array(
            'func' => function ($fields) {
                return '<a href="' . site_url('applications/pdf/'. $fields['id']) . '" rel="noopener noreferrer" target="_blank">VIEW</a>';
            }
        ),
        'EDIT',
        'DELETE'
    )
);
When I goto my module's list page I don't see the PDF label, instead it is VIEW.
Comments
The link text in your return statement says "VIEW". Change that to PDF.
-_- thanks didn't see that