XX is damaged and can't be opened

Before I get numerous answers telling me to change the settings in Security, please note that this problem relates to apps, PDFs (whether downloaded or created by myself), JPGs AND tiffs.

I successfully migrated all my information from my old MacBook Pro to my spanking, brand new MacBook Pro and now I have a MAJOR problem.

Every time I try to open one of the above files I get the following message: "XXX is damaged and can’t be opened. You should move it to the Trash."

I can open JPGs in Photos, but if I move it to the Desktop, same problem. The problem exists in all accounts on the computer. All the files can be opened on other devices.

I have re-installed High Sierra, repaired permissions and basically done everything suggested in forums and what Apple Support has said up to now. I'm still in contact with them, but I thought I'd throw the problem out there to see if anyone has any ideas.