You double-click a downloaded file and an unexpected application opens. The default association may explain the behaviour, but changing it is not the same as checking whether the file itself is safe.
First confirm what you downloaded, where it came from and which file type it claims to be. Do not rely only on the icon or the friendly name displayed in a message. Treat an unexpected executable or unfamiliar attachment cautiously and follow your organisation’s security process where applicable.
Inspect before setting a permanent default
Microsoft’s default-app guidance explains that Windows can assign applications by file type or protocol. In Settings, Apps, Default apps, you can look up a particular extension and review the selected application.
Choose software you know is suitable and obtained through a trusted route. A website’s insistence that you need an unknown “viewer” is not a reason to install it. If you only want to test an established alternative, use the operating system’s supported open-with option without changing every future association unnecessarily.
For example, a plain text file might open in a basic editor when you prefer another installed editor. That is a usability choice. A file that only pretends to be a document requires a different security decision.
Keep content and application separate
A familiar app does not guarantee that a file is harmless, and an unfamiliar icon does not prove that it is malicious. Keep software updated, retain appropriate security protections and avoid enabling active content simply to dismiss a warning.
After a legitimate change, test with a known harmless file of the same type. Record the previous default if you may need to restore it. The useful outcome is predictable opening behaviour, without letting a convenient double-click replace basic judgement about the download.