To enable the Document ID Feature
1. Go to Site Settings > Site Collection Features and Activate the Document ID Service.

2. Or you can use the following SharePoint 2010 Management Shell cmdlet to Activate the Document ID Service.
Enable-SPFeature -id docid -url <site collection url>
3. To customize the Document ID and apply Document ID to all existing Documents in the Site Collection. Go to Site Settings > Document ID Settings
And provide the String with which the Document ID will begin. I have provided Apollo for this example. Click Ok.

4. Now if you go to your document properties you should see, it has a new property called Document ID followed by a Unique ID for that document

5. The above document now has a unique ID and we can use the following link to locate the document no matter where the document is in the Site Collection
6. Note the Document ID’s might not show up instantaneously after activating the Document ID Service, the reason behind this is it creates a Timer Job for the same and once the job is executed by SharePoint you should start seeing the Document ID Field with those Unique ID’s, But if you can’t wait you can manually Run the Document ID assignment job under Central Administration > Monitoring > Review Job Definitions, or if you want to go the old school style you can run the following SharePoint 2010 Management Shell cmdlet to trigger the same.
Start-SPTimerJob -Identity DocIdAssignment
As we can see with Document ID locating and linking your document in SharePoint 2010 is easier than ever.