You need to update your observer function's code :
public function execute(\Magento\Framework\Event\Observer $observer)
$_product = $observer->getEvent()->getProduct();
$_sku = $_product->getSku();
$writer = new \Zend\Log\Writer\Stream(BP . '/var/log/Rohan.log');
$logger = new \Zend\Log\Logger();
It is naming convention issue. Please rename your controller action PDF to Pdf and run setup:upgrade and di : compile. And check route http://store/admin_1evvgb/testpakke/download/pdf
Hope it will help you.