With the letter that you receive from the Commission informing you that you have been selected to undergo a Horizon2020 EU Audit, there is an attachment that contains a list of the required supporting documents that need to be available before and during the audit.
It is a long list, but do not panic, go through it one at a time to create a folder that has all the information ready for the auditors.
To view the H2020 Letter of Announcement Annex 1 Attachment, please click here.