Hi Pauld,
I checked that you have Adobe PDF Pack subscription. The service is active and working fine.
The subscription plan does not need a serial number to activate the service. You just need to log in with your Adobe ID.
Assuming you are using Adobe Reader DC to export the PDF to Word file, try to sign out of the application.
Then relaunch the application and log in with your Adobe ID under which you have purchased the subscription.
Now try to export PDF and check if it works.
You can also export file online here https://cloud.acrobat.com/exportpdf
Open the link and log in with your Adobe ID.
Then go to Export PDF tab and follow the steps.
Please make sure that you are using the same Adobe ID under which you have purchased the subscription.
Can you share the Adobe ID in a private message so we can check it?
Note: Please do not share your Adobe ID in this forum post or any other social sites as it can be misused.
Refer this thread How Do I Send Private Message for the steps to send a private message.
Let us know if you still experience any issue.
Regards,
Meenakshi