To be able to open encrypted documents with eID, you must have your electronic identity card (and your pin code) and an eID card reader.
Once these conditions are met, you can link your eID to your Doccle account. See How can I link my electronic identity card (eID) to my Doccle account?
Once your eID is linked to your account, you can also use it to log in to your Doccle account, open encrypted documents and encrypt other documents yourself.