Most EU documents are normally released after 30 years, under the rules for historical archives. Many documents can be studied in the historical archives in Florence, close to the European University Institute.
The European Parliament has its own historical archives in Luxembourg.
Links
http://www.eui.eu/ECArchives/EN/
Since 1992 they have been handled through the "Archis" database: http://ec.europa.eu/historical_archives/archis_en.htm