cols[1] = {text: 'Right part', alignment: 'right', margin:[20,0] };

buttons: [ {
            extend:    'pdfHtml5',
            exportOptions: {
                columns: [ 0, 1, ':visible' ]
            },
            titleAttr: 'PDF',
            customize: function(doc) {

                var cols = [];
                cols[0] = {text: 'COPYRIGHT © 2017', alignment: 'left', margin:[20,0] };
                cols[1] = {
                    text: [
                        { text: page.toString() },
                        ' of ',
                        { text: pages.toString() }
                    ],
                    alignment: 'right', margin:[20,0] };
                var objFooter = {};
                objFooter['alignment'] = 'center';
                objFooter['columns'] = cols;
                doc["footer"]=objFooter;

            }
    } ]