Using 2 different currencies inside invoice.

Hi!
You can do this by filtering the prices, add the following to your theme functions (read this if you haven’t edited functions.php before):

add_action( 'wpo_wcpdf_process_template_order', 'wpo_wcpdf_before_pdf', 5, 2 );
function wpo_wcpdf_before_pdf ( $template_type, $order_id ) {
    add_filter('wc_price', 'wcpdf_add_bgn_amount', 10, 3);
    // echo '<pre>';var_dump($template_type);echo '</pre>';die();
}

function wcpdf_add_bgn_amount( $formatted_price, $price, $args ) {
    extract( apply_filters( 'wc_price_args', wp_parse_args( $args, array(
        'ex_tax_label'       => false,
        'currency'           => '',
        'decimal_separator'  => wc_get_price_decimal_separator(),
        'thousand_separator' => wc_get_price_thousand_separator(),
        'decimals'           => wc_get_price_decimals(),
        'price_format'       => get_woocommerce_price_format()
    ) ) ) );

    // conversion rate
    $bgn_rate = 1.95802248;
    // convert EUR to BGN
    $bgn_price = $price * $bgn_rate;

    $negative        = $bgn_price < 0;
    $bgn_price       = apply_filters( 'raw_woocommerce_price', floatval( $negative ? $bgn_price * -1 : $bgn_price ) );
    $bgn_price       = apply_filters( 'formatted_woocommerce_price', number_format( $bgn_price, $decimals, $decimal_separator, $thousand_separator ), $bgn_price, $decimals, $decimal_separator, $thousand_separator );

    if ( apply_filters( 'woocommerce_price_trim_zeros', false ) && $decimals > 0 ) {
        $bgn_price = wc_trim_zeros( $bgn_price );
    }

    $formatted_bgn_price = sprintf(' <span class="amount">(%s BGN)</span>', ( $negative ? '-' : '' ) . $bgn_price);

    return $formatted_price . $formatted_bgn_price;
}

Hope that helps!

Ewout