Search in Pages will not search documents

If by search in the upper right, and you mean the Find toolbar item on Pages, then it doesn't find files, only stated content in your existing document.

If you mean the Spotlight 🔍 search from the main menu bar, you first need to have selected the Spotlight Search Result categories that can be indexed (e.g. Documents in the case of Pages), and then force indexing of your startup drive. If you have done that, then there are reserved words used in Spotlight that can help you Narrow your search results on Mac - Apple Support.

For instance, let's say you want to find every Pages document that you have created since June 30th whose name contains the case-insensitive string "work" and has a red Finder tag on it (in non-German speaking countries):

kind:pages created:>06/30/2024 name:work tag:red

You then down-arrow to the resulting file and press the cmd-key to see its location, or cmd+R to open a new Finder window with that file selected, or just press return to open that Pages document in the Pages application.