Custom button to download a PDF

Custom action into subpanel row actions - Add the button to metadata

// file: custom/modules/<module>/clients/base/views/supanel-list/supanel-list.php
<?php
$viewdefs['Calls']['base']['view']['subpanel-list'] = array(
    'template' => 'flex-list',
    'favorite' => true,
    'rowactions' => array(
        'actions' => array(
            array(
                'type' => 'rowaction',
                'css_class' => 'btn',
                'tooltip' => 'LBL_PREVIEW',
                'event' => 'list:preview:fire',
                'icon' => 'fa-eye',
                'acl_action' => 'view',
            ),
            array(
                'type' => 'rowaction',
                'name' => 'edit_button',
                'icon' => 'fa-pencil',
                'label' => 'LBL_EDIT_BUTTON',
                'event' => 'list:editrow:fire',
                'acl_action' => 'edit',
            ),
            array(
                'type' => 'unlink-action',
                'icon' => 'fa-chain-broken',
                'label' => 'LBL_UNLINK_BUTTON',
            ),
            // custom option
            array(  
                'type' => 'rowaction',
                'icon' => 'fa-eye',
                'event' => 'list:pdfbtn:fire',
                'label' => 'LBL_PDF_BUTTON',
            ),
        ),
    ),
    'last_state' => array(
        'id' => 'subpanel-list',
    ),
);

Custom action into subpanel row actions - Add the label to language

// file: custom/Extension/modules/<module>/Ext/Language/en_us.customPdfButton.lang.php
<?php
$mod_strings['LBL_PDF_BUTTON'] = 'PDF';

Custom action into subpanel row actions - Handle click event

// file: custom/modules/<module>/clients/base/views/supanel-list/supanel-list.js
({
    extendsFrom: 'SubpanelListView',
    // adding the listener to custom button event
    contextEvents: {
        "list:editall:fire": "toggleEdit",
        "list:editrow:fire": "editClicked",
        "list:unlinkrow:fire": "warnUnlink",
        	"list:pdfbtn:fire": "downloadPdf"
    },
    
    /**
     * Handle click on custom button
     */
    downloadPdf: function(model) {
        // TO DO 
    		console.log('Helo!!!');
    },
})

I suggest to use the function: app.api.fileDownload.
Function definition:
/**
* Given a url, attempts to download a file.
*
* @param {string} url url to call
* @param {Object} [callbacks] Callback object
* @param {Object} [options] Options object
* - iframe: jquery element upon which to attach the iframe for download
* if not specified we must fall back to window.location.href
* @memberof Api
* @instance
*/
fileDownload: function(url, callbacks, options)

How to use it:
app.api.fileDownload(url, {
error: function(data) {
app.error.handleHttpError(data, {});
}
}, {
iframe: this.$el
});