alimg (./2) :
I solved the problem myself. The best way I deal with this problem is including the content of the CSS file inside my content_html before converting it to PDF.

the code is:

$content_html = "<style type='text/css'>".file_get_contents("css/pdfReport.css")."</style>".$content_html;

Yes, I've found this is the best way to do it. Inline styles seem to be touchy, especially padding & margins, so I try to get everything in the css file.