Css不适用于pdf文件，适用于html

<?php 
$this->fontdata = array(
    "HelveticaNeue" => array(
    'R' => "HelveticaNeue.ttf",
    'B' => "HelveticaNeueBold.ttf",
    'I' => "HelveticaNeueItalic.ttf",
    'BI' => "HelveticaNeueBoldItalic.ttf",
     ),
 );
 ?>
<h6 style="margin-top: 8px;margin-bottom:8px;font-family:HelveticaNeue;">
    <small>Device Location :<b><?php echo $device_location; ?> </b></small>
</h6>

in html file device location appear bolt. But when i am converting the same page into m pdf device location is not appear bolt.