Hello,
You can try to encrypt the generated PDF. Here are the relevant API settings (PHP syntax):
$client->setNoPrint(True)
$client->setNoModify(True)
$client->setNoCopy(True)
$client->setOwnerPassword("password");
The above will allow a user to open the PDF for viewing without a password but he/she won't be able to print, modify, or save/export it. If you want, in addition, to protect the PDF from viewing with a password, add this line:
$client->setUserPassword("password2");