Apple does not keep past full Pages installers for download, and as you have observed, Pages 7.1 has newer operating system restrictions. Unless you have Pages v5.6 on a Time Machine backup somewhere, you are done with Pages (see below).
Does it need to be Pages? There are plenty of non-Apple alternatives both in the Mac App Store, or even the free LibreOffice, which is an MS Office clone.
Did you ever obtain an iWorks '09 DVD in the past? It you did, you can install that on El Capitan, and then update it with the Apple 9.3 iWork '09 updater. If you ever purchased Pages '09 from the Mac App Store prior to Oct. 2013, you can simply drop that into the /Applications folder, but you cannot use the preceding updater on it. Pages '09 cannot directly open Pages v5,6, or 7 documents.
Amazon and Ebay resellers offer retail iWork '09 DVD for sale. Price may exceed Mac App Store alternatives.