The file you are trying to preview could damage the computer if you trust the file

Hello, I have had problems opening pdf files in dropbox, desktop version for denied access.
I have given it permissions from preferences / enhanced security and deactivating protected mode.

These files, I can not open them in preview from the file browser.

The following message appears:
the file you are trying to preview could damage the computer if you trust the file.

What can I do to see it in the preview from the file browser?