It looks like I have to experiment a little with the the DPI-settings, but it works! Thanks a lot!

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Experimenting a little, this even let's me use the external css-file (which is great because it's also used for the ckeditor):

$htmlbody ='<style type="text.css">
    <!--
    '
    . str_replace("\"","", str_replace("url(\"", "url(\"".dirname(__FILE__). "/", file_get_contents("contentOA.css")))  . '
    -->
    </style><page>'.$htmlbody.'</page>';

Thanks a lot!!!