Hi Nigels,
As shared, does it ask for the login information or subscribe for the subscription? How exactly you are trying to access the service?
You may follow the steps suggested below, and see if that helps-
1- Export PDF is an online service that you can use online via: https://cloud.acrobat.com/exportpdf
2- You can also use the services through free desktop version of Adobe Acrobat Reader DC: Adobe Acrobat Reader DC Install for all versions
3- “Sign In” using the email (Adobe ID) under which you have subscription registered.
4- For detailed description of how to use the Export PDF service, you may refer to- Using Adobe Export PDF
You may refer to the following link for any troubleshooting: Troubleshooting Create PDF, Export PDF, Adobe Send, Fill & Sign … services(PDFPack or Document Cloud Services).
Let us know if it still doesn't work.
Regards,
Akanchha