Recipient can't read attachments in Pages

What is your general goal for the document here? Sending a document, or collaborating with the recipient?

Usual format for mailing around documents is PDF. Pages can export to that, and you can attach and mail that. All Apple platforms, Microsoft Windows, and most other platforms all have a PDF viewer.

Here is how to share and to collaborate documents using Pages, including if the recipient doesn’t have Pages:

… Collaborate on documents in Pages, Numbers, and Keynote on iPhone, iPad, and Mac - Apple Support

They will need a web browser, but will not need a Mac or any other Apple device, or an Apple ID:

… Intro to collaboration in Pages for iCloud - Apple Support

If the recipient has Word, Pages can export to that and mail that.

Apple has an intro to Pages available:

… Intro to Pages on Mac - Apple Support

Short answer is the only application in the world that can open a Pages document is Pages, and that means the recipient must have an Apple device with the appropriate version of Pages on it, or the ability to use Pages in iCloud.

If the recipients purpose is to only read, then send a PDF, and if otherwise unknown, send an exported Word document. Since Pages is not a Word clone, what you see in Pages before exporting to Word may appear differently to the recipient opening that Word document in Microsoft Word, or a Word clone.