Replace the configurable product’s qty with that of the simple product's qty

Edit Mypdf.php

insert

$customPdfTemp = array();
foreach ($printItem->getAllItems() as $item) {
    if ($item->getQtyBackordered() > 0) {
        $customPdfTemp[] = array('qty_backordered' => $item->getQtyBackordered(), 'item_id' => $item->getItemId(), 'parent_item_id' => $item->getParentItemId());
    }
}

before

foreach ($printItem->getAllItems() as $item) {

and

foreach ($customPdfTemp as $key => $value) {
    if ($value['parent_item_id'] == $item->getItemId()) {
        $item->setQtyBackordered($customPdfTemp[$key]['qty_backordered']);
    }
}

after

$pdfTemp = array();