The what? Well, few people used it which is why it is shutting down; but the Adobe Document Center is a service for protecting documents, somewhat similar to Microsoft’s Rights Management Services except that it is provided as a hosted subscription service; though I am not sure that it ever made it out of beta and actually started charging. You can use it with a PDF or Microsoft Office document to restrict who can access it and set an expiry date.

At least, you could. I have received an email (because I must have tried the beta back in 2006) informing me that the service is shutting down on April 2nd 2011:

Important: This means that after the Service shuts down you, or anyone you have distributed documents protected via the Service, will no longer be able to open/access these documents. We strongly encourage you to use Adobe Acrobat to un-protect these documents before the Service is shut down.

Time to make a mental note: protected documents are high-maintenance and there is always a risk of losing your data.