I am getting this downloaded sometimes.

    $report->run()
        ->export('Executive_summary')
        ->pdf(array(
            "format" => "A4",
            "orientation" => "portrait",
            //"zoom"=>2
        ))
        ->toBrowser("executive_summary.pdf");

If I try to open directly from the hard drive I get this:

Any idea why?

you should post all the code. you are only posting the export code snippet.

no one can answer these questions without the code that ties into this. What does your executive_summary look like? If you truly want help from The Team and form others on here, all I can say is post way more detail.