I have a 2010 iMac. One day I installed the "recommended security updates" and all of a sudden I had High Sierra installed on my iMac instead of El Capitan.
Now I cannot not open any pdf, jpeg png's and more. I think my preview app might be broken. I always get the message "x is damaged and cannot be opened. It should be moved to trash".
I have tried using the command
sudo spctl --master-disable in terminal and turning off Gatekeeper but it isn't helping.
Apart from this problem the my Mac is running decently. We do have time machine backups to go back to when it was on El Capitan.
How can I fix this?