In your internet browser preferences, look for the option, “open files automatically after downloading” and make sure it is OFF.

This is a Safari example.

There are no “safe files”.

Opening a file is what unleashes the problems.

Those problems (malware, virus, trojan horse, whatever) can be hidden within a photo, text document, music file, and especially a PDF.

The Attack:

Visit a webpage > a file is downloaded without your consent > that box is checked > file opens > computer is compromised

The Defence:

Any service that allows you to download a file, should have this setting OFF.  Skype, chat program, email program…

Therefore, even if the file downloads automatically, you can then delete it from the downloads folder, without opening it.