Apple Pages, like Microsoft Word, is a full-featured document creation tool. Because you can use it for free on macOS, iOS, and iPadOS, this makes it a solid choice for creating reports, contracts, ...
You can convert Pages to PDF on a Mac using the default exporting method to make sharing with non-Mac users much easier, for instance.
To change the default PDF viewer on Windows 10, you'll need to edit your default apps in the Control Panel.