class PDF extends FPDF {
	private $surface='';
	private $data='';

	//initialise les variables globales
	function InitVar($item) {
		$this->surface 	= trim ($item->surface); 
		$this->data = JRequest::get('post');
	}

function printy()
	{          
		// Get data from the model
		$model = $this->getModel(batenergieitem);
		$model->activeAll();
		$model->active('predefined', 'default');
		$item		= $model->getItem();
    
           		require(JPATH_COMPONENT.'/print/pdf_file.php'); }
          
		$pdf->Output();
	}

$this->data = JRequest::get('post');

require(JPATH_COMPONENT.'/print/pdf_file.php');

require(JPATH_COMPONENT.'/print/pdf_file.php'); //same
//Then :
$pdf->InitVar($item);	//Here you send your item to the class (below, to code to change in your PDF class)
$pdf->Output();

class PDF extends FPDF {
	private $surface='';
	private $data='';


//When is called this function ? (At the contruct, or before Output() ?? VERY IMPORTANT)

	//initialise les variables globales
	function InitVar($item) {
		$this->data = $item;
		$this->surface 	= trim ($this->data->surface); 
	}