There are three different kinds of letters that can be made available for download by participants:
Confirmation letters for authors including an invitation to participate
VISA invitation letters
Confirmation letters for participants, confirming their attendance
All three letters can be configured here:
Overview > Settings > Settings for Confirmation Letters
In this particular case, go to the segment “Confirmation Letters for Participants, Confirming Their Attendance” and set the phase according to your needs. Check the checkbox “Activated in above period” to enable this functionality. Furthermore, you can limit the download possibility to those people who checked in at the front desk of the conference in the section “Conditions to access the confirmation of participation”.
NB: As the administrator you will always be able to download the corresponding letters when you open a user’s profile.