Provided you have installed Pages, Numbers, and Keynote before, launch the Mac App Store from the Dock, sign in with the same Apple ID used to previously obtain these applications. Then press cmd+0 (zero) to open the Purchased panel, and click the update button for Pages. A dialog will appear informing you that you need a newer version of macOS¹. Agree, do not select Cancel, and that dialog will hopefully be replaced with another dialog offering to install the latest compatible (v11.1) version of Pages for macOS Catalina. Repeat this process for Numbers and then Keynote.
If that paragraph fails to obtain Catalina compatible versions of these applications, then you can use the iCloud versions of these applications, or their current App Store installations on iOS/iPadOS 15.*. Apple does not provide full installers for past versions of these applications. Only the relevant Apple applications mentioned here can open their respective document formats, so there is no other recommended application to open them. If you have no legacy Apple applications, then I suggest that you trial the Office 2021 for Mac from SoftMaker. Easier to use than the commonly recommended free LibreOffice Suite, and at least as compatible in document formats as MS Office.
¹ One now needs macOS Big Sur or later for the current (v11.2) version of Pages, Numbers, and Keynote.