I have one SP site where there is an event receiver that gets executed and keeps track of all the information about the documents which are getting downloaded.
Business plans to make this site read only soon, so I am worried if users download the documents post read only, then will the event receiver trigger and try to push the details of the downloaded documents in SP List?
It depends how way you want to set read-only mode.
You can set with PowerShell Set-SPSite -Identity "<SiteCollection>" -LockState "<State>" or you can set Permissions on this Site, so you will set for every users Read Only Permissions.
If you set Read-Only Permissions for all, administrator will always have Full Control permisisons and the Event Reciever runs under Admin, so it should work correctly.
If you set LockState, there could be problem because neither Administrator could not do any changes on this Site.
