But the pdf is cutting part of my report, so I suppose that I have to provide additional information (max width of my table report for example) somewhere.

It is probably linked to the size oy your tables or divs.
If you set size in px or mm or eml, it may be larger than page size
If you set size it %, it should auto adjust to page size.

I would like to use the portrait mode for my new bill, but not for other PDF exports, so is it possible to define it somewhere?

No, you'll have to have code it, something like :

if (myReport) $html2pdf = new HTML2PDF('P','A4','en'); else $html2pdf = new HTML2PDF('L','A4','en');

Here it is what I've done in the print.php file.

/* JPH added */
// $html2pdf = new HTML2PDF('L','A4','en');
$myReport = $_REQUEST 'reportFile' ;
if ($myReport == 'bill-jph.php') {
$html2pdf = new HTML2PDF ( 'P', 'A4', 'en', true, 'UTF-8', array(5, 18, 5, 10) );
} else {
$html2pdf = new HTML2PDF ( 'L', 'A4', 'en' );
}
/* end JPH */

And it works as I want.
But this is not the best way of doing it, on the next upgrade I will have to remodify the file.
For the bill, I've created a new file and modified into the database the call for the bill to my new file, so upgrading will remain easy.
I don't know well enough ProjeQtor to make a suggestion on how to allow customisations to remain compatible with core upgrades, but I would have found better if I could store the HTML2PDF arguments within the report file.

Thank you very much, anyway.
Ciao
Jean Pierre

I don't know well enough ProjeQtor to make a suggestion on how to allow customisations to remain compatible with core upgrades

Yes, IT is a real constraint.
Not sure there is an easy clean solution.
Any suggestion is welcome.