Why do PDF files from my own server open in a browser window instead of in Preview like other PDF files?

The issue may be due to the way the PDF files are being served from your server. To force the PDF files to download instead of opening in a browser window, you can set the Content-Disposition header to attachment in the server response. 

header('Content-Type: application/pdf');
header('Content-Disposition: attachment; filename="example.pdf"');
readfile('path/to/your/pdf/file.pdf');

Keywords

PHP PDF server configuration MIME type browser settings

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