Is it possible for a file virus to execute without user knowledge?

The tricky part is that even if a file has a hidden virus, you need something to trigger it. A virus can't just activate on its own when you open a regular document. It requires an executable file to run and exploit any vulnerabilities in the software you're using. So while your idea is interesting, it doesn’t quite align with how files or viruses operate in practice.

Websites can't directly open files on your computer. They can only host downloadable files. Even if a user downloads an unknown format, the computer will prompt them to choose how to open it. The best way to spread malware is to have users download and install a program that you’ve crafted to include the virus as its part. Sometimes, smaller bugs in these legitimate programs can accidentally open doors for bad code without users even realizing it.

Actually, not quite how it works. All files, whether it's a text or a PDF, are just different types of data. You could rename a .exe file to .txt, but it’s still an executable file. If you open an unknown file type, your computer usually prompts you to download it or select a program to open it. However, if a program you install has vulnerabilities, that's where malware might sneak in. So creating a deceptive file isn’t the best approach for distributing a virus.