Hello,

I noticed a problem.

When changing the page size:

$customPaper = [0, 0, 800, 1100];
$domPdf->setPaper($customPaper);

and then calling render() :

$domPdf->render();

Dompdf recreates the canvas and replaces the existing one with the one containing embeddedFile.
The relevant code is in src/Dompdf.php at line 766. The if at line 765 is true because the width and height specified in $customPaper do not match the default dimensions (612 width and 792 height).

I worked around this issue by doing a pre-render to "lock" the canvas with the custom size, and only then adding the embedded file:

private function preRenderCanvasDomPdf(): Dompdf
{
    $options = new Options([
        'isRemoteEnabled' => true,
        'isPdfAEnabled'   => true,
        'defaultFont'     => 'DejaVu Sans',
    ]);

    $domPdf = new Dompdf($options);

    $customPaper = [0, 0, 800, 1100];
    $domPdf->setPaper($customPaper);
    $domPdf->loadHtml($yourTemplate);

    $domPdf->render();

    return $domPdf;
}

$dompdf = $this->preRenderCanvasDomPdf();

$cpdf = $dompdf->getCanvas()->get_cpdf();

$cpdf->addEmbeddedFile(/* ... */);

This is the only reliable way I’ve found without touching the library itself.
If anyone’s motivated, a PR would be more than welcome

I’ve tried to make my explanation as clear as possible, but please feel free to give me feedback if anything isn’t clear