fPDF

add_action( 'wpcf7_before_send_mail', 'save_application_form');
function save_application_form($cf7) {

/* GET EXTERNAL CLASSES */
require(get_stylesheet_directory() . '/fpdf/fpdf.php');

$pdf = new FPDF();
$pdf->AddPage();
$pdf->SetFont('Arial','B',16);
$pdf->Write("Hello, World! Is there anything below this?");
$pdf->Output(get_stylesheet_directory() . '/pdfgen/pdf.pdf', 'F');
$cf7->uploaded_files = array ( 'attachedfile' =>  get_stylesheet_directory() . '/pdfgen/pdf.pdf' );
}