... is damaged and can't be opend.

[keessmoors]

If i open any file it doesn't matter which kind of file, it will always tell me that it's damaged and can't be opend. I already tried to reboot my laptop a few times but that doesn't fix the problem. And now comes the funny part, if i open a file within a program let's say preview it does open. And then i trie to open it just by double clicking on in and it says again that it is damaged. How can i fix this problem??

I have a Mackbook air running Mac OS High Sierra 10.13.2

 

[pascable]

Go to system preferences.
-> "Users and groups".
-> Create a new admin with any name and password.
Restart on this session and see if you have the same problem.
This will give you an idea if it is your system that is in question or rather an electronic problem, damaged disk for example.
There is a good chance that it is your system.

 

[ZapNZs]

Are all files backed up?

Have you already tried booting into the recovery partition and repairing the disk with Disk Utility?
Have you tried booting into safe mode and seeing if the problem persists?
(While unlikely to have an affect) have you reset the PRAM/SMC?
Have you performed the Apple Diagnostics to check for hardware issues?