Information on how to attach supporting documents in the Single Candidate Portal is available at How to set up your profile and your CV | EU Careers (in particular fig. 4, 6 and 8).

Please note that these documents do NOT need to be uploaded and linked in your EPSO Account. All supporting documents must be attached to the CV of your Single Candidate Portal profile at the latest by the deadline mentioned in the Notice of Competition. 

For more information please read: Which supporting documents should I provide?