Let us recall Directive 2002/65/EC of the European Parliament and of the Council of 23 September 2002 concerning the distance marketing of consumer financial services and amending Council Directive 90/619/EEC and Directives 97/7/EC and 98/27/EC, which defines “durable medium” as: “any instrument which enables the consumer to store information addressed personally to him in such a way that he can easily retrieve it over a period of time adequate for the purposes for which the information is intended and which permits the unchanged reproduction of the information stored”.
The predecessor is Directive 97/7/EC of the European Parliament and of the Council of 20 May 1997 on the protection of consumers in respect of distance contracts, which mentions the “durable medium” requirement without defining it.
The lack of specificity in that rule led to the judgment of the Court of Justice (Third Chamber) of 5 July 2012, which ruled: “A medium must be considered “durable” within the meaning of that provision insofar as it enables the consumer to store the information addressed personally to him, ensures that its content has not been altered, guarantees its accessibility for an appropriate period and offers consumers the possibility of reproducing it in an identical form”.
This approach is confirmed by the definitions of the concept of “durable medium” given by the EU legislator in other regulatory texts, in particular,

Article 2(f) of Directive 2002/65/EC, Article 2(12) of Directive 2002/92/EC and Article 3(m) of Directive 2008/48/EC.
Even if those directives are not applicable in the present case, there is no reason to consider that they refer to a concept other than that used in Directive 97/7/EC, as pointed out by the Advocate General in point 36 of his Opinion.
This is all the more true for Directive 2011/83/EC, which will replace Directive 97/7/EC as of 13 June 2014, and which defines, in Article 2, point 10, the concept of “durable medium”, according to the criteria cited in the previous paragraph.