read out a user's company name in onBeforeDownload

You can get the current user id like that:
$user_id = hikashop_loadUser();

Then you need to load the addresses of the user like that:
$class = hikashop_get('class.address');
$addresses = $class->getByUser($user_id);

Then, the first address:
$address = reset($addresses);

Then the company name:
echo $address->address_company;