Create PDf on woocommerce_thankyou page

Hello,
I want to create a pdf file from an html file and email it to the user by the woocommerce_thankyou hook.

Here is some of my code in function.php:

add_action('woocommerce_thankyou', 'send_thank_you_email', 1, 1);
function send_thank_you_email($order_id)
{
    $order = wc_get_order($order_id);
    $order_email = $order->get_billing_email();
    $first_name = $order->get_billing_first_name();
    $last_name = $order->get_billing_last_name();

    $_req_res = _send_email_voucher($_booking_number, $_hotelname_en,
        $order_email , $first_name, $last_name);


    $order = wc_get_order($order_id);
.
.
.

And in _send_email_voucher function I use TCPDF to create PDF from an HTML file.

I use TCPDF beacuse the HTML file has Persian utf-8 characters, and TCPDF supports Persian characters.

function _send_email_voucher($_booking_number, $_hotelname_en, $_hotelname_fa, $_roomType_en, $_roomType_fa,
                            $_chekin_en, $_chekin_fa, $_chekout_en, $_chekout_fa, $_passengers, $_hotelstar,
                            $_hoteladdr, $_hotelphone, $_xtra_services, $_to_address, $_to_name, $_subject = "???? ???? ???",
                            $_depart_flight_number, $_depart_date_time, $_depart_flight_aircraft, $_depart_flight_airline,
                            $_return_flight_number, $return_date_time, $_return_flight_aircraft, $_return_flight_airline
){
    ob_start();
    include("voucher.php");
    $_body = ob_get_contents();

    $pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);
    $pdf->setCreator(PDF_CREATOR);
    $pdf->setAuthor('Nicola Asuni');
    $pdf->setTitle('TCPDF Example 018');
    $pdf->setSubject('TCPDF Tutorial');
    $pdf->setKeywords('TCPDF, PDF, example, test, guide');
    
    // set default header data
    $pdf->setHeaderData(PDF_HEADER_LOGO, PDF_HEADER_LOGO_WIDTH, PDF_HEADER_TITLE.' 018', PDF_HEADER_STRING);
    
    // set header and footer fonts
    $pdf->setHeaderFont(Array(PDF_FONT_NAME_MAIN, '', PDF_FONT_SIZE_MAIN));
    $pdf->setFooterFont(Array(PDF_FONT_NAME_DATA, '', PDF_FONT_SIZE_DATA));
    
    // set default monospaced font
    $pdf->setDefaultMonospacedFont(PDF_FONT_MONOSPACED);
    
    // set margins
    $pdf->setMargins(PDF_MARGIN_LEFT, PDF_MARGIN_TOP, PDF_MARGIN_RIGHT);
    $pdf->setHeaderMargin(PDF_MARGIN_HEADER);
    $pdf->setFooterMargin(PDF_MARGIN_FOOTER);
    
    // set auto page breaks
    $pdf->setAutoPageBreak(TRUE, PDF_MARGIN_BOTTOM);
    
    // set image scale factor
    $pdf->setImageScale(PDF_IMAGE_SCALE_RATIO);
    
    // set some language dependent data:
    $lg = Array();
    $lg['a_meta_charset'] = 'UTF-8';
    $lg['a_meta_dir'] = 'rtl';
    $lg['a_meta_language'] = 'fa';
    $lg['w_page'] = 'page';
    
    // set some language-dependent strings (optional)
    $pdf->setLanguageArray($lg);

    $pdf->setFont('dejavusans', '', 12);
    $pdf->AddPage();
    $pdf->setRTL(true);
    $htmlpersian = '<span color="#660000">Persian example:</span><br />???? ??????? ???? PDF ????? ?? ??? ???? ?? ??. ???? ?? ?????.<br />???? ??? \"?\" ?? ???? ????? ????? ???? ???? ??? ?? ??? ??.<br />????? ???? ??? ? ??? ??? ?? ?? ??? ????? ??.<br />?? ???? ??  "Asuni Nicola" ? ???? ??? ?? ??? ???? ???????? ???? ?????.';
    $pdf->WriteHTML($htmlpersian, true, 0, true, 0);

    $pdf->Output('example_018.pdf', 'I');
            

.
.
.

But I get the following error:

TCPDF ERROR:?Some data has already been output, can’t send PDF file

For solving problem I use ob_start, But the problem still persists.

I know taht the problem is in the Thankyou Page already been output, what I don’t know is ?ow do I make my function “send_thank_you_email” the first thing to be executed?

Or is there any other way to Email this PDF file without problem?