More than likely the script is hitting a fatal error and exiting
before execution is complete. You need to figure out why the script is
failing. Do you receive any PHP errors (look at the PHP error.log)?
Just as an FYI, the error you are receiving is an operating system
error. Since the script dies early your web server is sending whatever
content it has. Since the failing page is dompdf.php, that is the file
sent to your web browser. More than likely the document you are
getting is plain text. Try saving it and opening it in a text editor
to see if any errors are shown.