mPDF error: Unable to create output file

Try not to modify the source. Instead, revert back to fopen($file_out,'wb'); and make sure that folder permissions are set to 0777, like you did.

Also, check if you're using the absolute path in order to write to a file, rather than using relative paths or even worse, URL.

So,

$file = "/var/www/html/mywebsite/mypdfs/myfile.pdf";

and try if it works.

Good luck!