How to restore Pages to an iMac

The last version of macOS that supports the iWork '09 applications is Mojave. You can only reinstall the Mojave-compatible versions of Pages, Numbers, or Keynote (not iWork '09 applications) if you previously installed them from the Mac App Store, and by using your Apple ID when that was done. If these applications were preinstalled when you took possession of the current Mac, then you cannot install them as the Apple ID would not be yours.

There are too many risk variables when attempting to purchase a used Mac running macOS Sonoma (14) from a third party. It may be stolen, or improperly prepared for resale, and that can create headaches for whoever purchases it.

I recommend that if you are planning the purchase of a new (or used) Mac, you get a refurbished Mac from Apple or Mac Sales.

The fact that you are getting that missing index.xml dialog tells me that you have an operational Pages '09 working on macOS Mojave (10.14.6), or an earlier version of macOS. Pages '09 cannot open documents created by Pages v5 or later as the document architecture is entirely different. That is the source of your dialog since you now have mixed architecture documents present. There is no means in the Finder to differentiate the old and newer Pages documents unless you assign different colored Finder tags to the documents that you know are in one of these architecture categories. I assigned a purple tag to Pages '09 documents and a yellow tag to Pages v5+.

Pages versions introduced after Oct 2013 could open both document architectures at the risk of converting the older format into the newer format and dropping unsupported document features. The newer applications also have an Export To > Pages 09 that filters newer features from being present in the exported documents — and these can be opened in Pages '09.

The seller of that other Mac needed to prepare it for resale so that the operating system and any previous applications installed were not tied to the former owner's Apple ID. If the purchased Mac does not boot to a configure screen where you choose the language, keyboard, and new user ID with a password, then it should be wiped and clean reinstalled before you begin using it. Then your Apple ID is associated with it.

From the Apple () menu > About This Mac, what does the product description line say about your Mac such as model year and make? Once that is known, I can direct you to the specific operating system to install.

That Late 2013 iMac came pre-installed with macOS Mountain Lion 10.8.4 and can be upgraded to macOS Catalina 10.15.7. From the Apple () menu, About This Mac panel, what version of macOS is installed now?

When you first power on the iMac, did it launch to allow you to select the language, user name, password, etc. initial setup information?

Try launching the Mac App Store and signing in with the same Apple ID that you used previously to install Pages. Then press cmd+0 (zero) to access the Purchased panel. If there is a button or icon present suggesting an update or download, do that. You will be presented with one or more dialogs. Do not click Cancel on these, and the last one will offer to install the last compatible version of Pages for your present operating system.

If your previous version of Pages is substantially out of date with the one expected for your given operating system, the first paragraph may fail until you move the existing Pages application to the Trash and empty it. Then repeat the first paragraph.