>”The supplied key does not work, the software asks for a 16-digit key, and the supplied one is only 8 digits long.”
That is incorrect. The setup dialog that mentions 16 characters is informing the user that their hardware key, which is 16 characters will be sent to their registration database. You enter the 8-digit key sent to you below that text. Here is the dialog text that is above the “Enter Product Key’ field:
The product key has been sent to you in the confirmation e-mail that you received after purchasing Perfect PDF Premium. During the registration process, your computer’s hardware key (16 characters) will be sent to our registration database. No other information about your system or about the files on your computer will be saved or sent. Registration will be carried out absolutely anonymously.
Enter the 8-digit key and the registration should work just fine.