Apple only keeps the most current version of Pages in the Mac App Store, and does not offer full downloads of past versions. The two iconic references are:
Pages v6.6.1 (macOS Sierra) Pages '09 v4.3
If you were using Pages v5.6.n (yellow icon) on El Capitan, and do not have it on a Time Machine backup, then you are out of luck, as it is no longer obtainable. If you were using Pages '09 v4 (purple icon) from the (retail) iWork '09 DVD, (or the Mac App Store) — it too is no longer available from Apple as it was discontinued in 2013.
If you do have a retail iWork '09 DVD (also available from Amazon and Ebay resellers), you can reinstall that into El Capitan, and then apply the Apple iWork 9.3 updater. That will bring the individual iWork '09 applications to their 12/2012 (last revisions). A trial version of the iWork '09 DVD is now worthless, and can be thrown away.
Pages v5 and v6 documents are not directly openable by Pages '09, and must be exported by their respective versions to Pages 09 format. If you have these on El Capitan, and without Pages v5.6.n to export with, you will not have access to these documents unless opened on a machine running Sierra or later.
Pages for Mac Support Community.