Trigger file download once a successfully submitted optin is closed.

OptinMonsterOptinSuccessClose.js

// Function SaveToDisk forces file download instead of viewing in browser
function SaveToDisk(fileURL, fileName) {
    // for non-IE
    if (!window.ActiveXObject) {
        var save = document.createElement('a');
        save.href = fileURL;
        save.target = '_blank';
        save.download = fileName || 'unknown';

        var event = document.createEvent('Event');
        event.initEvent('click', true, true);
        save.dispatchEvent(event);
        (window.URL || window.webkitURL).revokeObjectURL(save.href);
    }

    // for IE
    else if ( !! window.ActiveXObject && document.execCommand)     {
        var _window = window.open(fileURL, '_blank');
        _window.document.close();
        _window.document.execCommand('SaveAs', true, fileName || fileURL)
        _window.close();
    }
}

jQuery(document).ready(function($){
    
    // Specify the OptinMonster Event that will close our optin
    $(document).on('OptinMonsterOptinSuccess', function(event, data, object){
        object.close();
    });

     // Specify the OptinMonster Event that will trigger our file download
     $(document).on('OptinMonsterOptinSuccessClose', function(event, data, object){

        // Update with the url of the file you want downloaded on optin submission
        SaveToDisk('http://www.example.com/download-file.pdf');

     });

});