Use of FPDF library in CI4

Hello guys,
As I need fpdf in my project, I have installed it thru seatsign/fpdf found in packagist.org, using :

composer require setasign/fpdf:^1.8

It is now present in the vendor\setasign\fpdf directory of my app

To get it in my project, I have put a file MyFpdfClass.php in \APP\Libraries containing :
<?php
namespace App\Libraries;
use \setasign\fpdf\fpdf;
[...]
class MyFpdfClass extends Fpdf
{

but the call in controller $t = new MyFpdfClass() send an error :

Class "setasign\fpdf\fpdf" not found
APPPATH/Libraries/MyFpdfClass.php at line 13

How can I set the "use" so that the library is found ?
Is it a problem with autoloader not correctly set ?

Thanks for your help
Eric