Download all Reports from SSRS reporting server

Install-Module -Name ReportingServicesTools
$sourceRsUri = 'http://reportginserver/Reportserver'
#Declare Proxy so we dont need to connect with every command
$proxy = New-RsWebServiceProxy -ReportServerUri $sourceRsUri

#Output ALL Catalog items to file system
Out-RsFolderContent -Proxy $proxy -RsFolder / -Destination 'c:\Backup_Reports' -Recurse

Users cannot view ACS reports.

For the ACS reports the permissions have to be configured in SCOM.

Click Administration--User Roles

If you do not have a role created. Create a new role called "ACS report viewers"

In the tab that opens add the groups in your AD or users in your AD that you would like to have permissions to view reports.