According to Article 7.3 of the General Conditions of Tender, for electronic bids, an advanced electronic signature supported by a qualified certificate is required. An advanced electronic signature based on a qualified certificate is an electronic signature that:
Advanced electronic signatures based on a qualified certificate fulfil the legal requirements for a signature with respect to data in electronic form in the same way as handwritten signatures with respect to data on paper.
Bidders are allowed to use any advanced electronic signature that is based on a qualified certificate offered by a certification service provider registered in the list of confidence in accordance with Commission Implementing Decision 2013/662/EU of 14 October 2013 amending Decision 2009/767/EC as regards the establishment, maintenance and publication of trusted lists of certification service providers supervised/accredited by Member States. See the following link http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32013D0662&rid=1
Trusted lists are available for consultation on the following links
a signed xml machine processable version,
a human readable version,
Further information can be found in Regulation (EU) No 910/2014 under the link above.
Furthermore, as an example, in case the signed document is opened in Adobe® Acrobat® Reader or any other PDF reader, other helpful information, including a list of certificate service providers who create digital signatures that are trusted according to the Commission Implementing Decision 2013/662/EU of 14 October 2013 amending Decision 2009/767/EC can be found on the following links