$pdfmerger->download() blocked by CORS

[chongyc1]

my code:

...

$oMerger = PDFMerger::init();
$oMerger->addPDF($path1, 'all');
$oMerger->addPDF($path2, 'all');

//for local success
return $oMerger->download();

I got this error from dev tabconsole
Mixed Content: The site at 'https://domain.com' was loaded over a secure connection, but the file at 'https://domain.com/print' was redirected through an insecure connection. This file should be served over HTTPS. This download has been blocked. See https://blog.chromium.org/2020/02/protecting-users-from-insecure.html for more details.

Please give any solution. Thanks.