The FNRS rules stipulate that Â«seules les publications en version Ã©lectronique intÃ©grale gÃ©nÃ©rÃ©es sous format pdf Ã partir des dÃ©pÃ´ts institutionnels des universitÃ©s de la FÃ©dÃ©ration Wallonie-Bruxelles seront prises en considÃ©ration par le F.R.S.-FNRS et les fonds associÃ©s dans leurs procÃ©dures dâ€™Ã©valuation et dâ€™attribution de crÃ©dits de rechercheÂ» (Article 9).
The FNRS requires that you upload on DI-fusion, the post-print of your articles. This is the version after peer review and after all revisions. (The pre-print is the version of the paper before peer-review.) In terms of content, post-prints are the article as published. However, in terms of appearance this might not be the same as the published article, as publishers often reserve for themselves their own arrangement of type-setting and formatting. Typically, this means that the author cannot use the publisher-generated PDF file, but must make their own PDF version for submission.
The FNRS invites researchers to deposit the post-print version (but the publisher-generated pdf is ok also if permitted by the publisher!), as soon as possible after publication, and no later than 6 months after publication. This rule applies to all publications starting from 30th June of 2013. For previous publications (before 30th of June, 2013), the FNRS invites you to deposit the articles published since 2008.
In order to know the open access policy of the publishers, please consult the Sherpa/Romeo database http://www.sherpa.ac.uk/romeo/ ; some publishers authorize deposit on open access of the postprints, sometimes with an embargo period, others authorize the deposit of the publisher-generated pdf.