Sandra,
If you still have that retail iWork '09 DVD, you will need to reinstall it again after the El Capitan upgrade, and then apply the Apple iWork 9.3 updater to get to Pages '09 v4.3 — a pre-requisite for El Capitan compatiblity. Note that I put emphasis on a retail iWork '09 DVD.
I have Pages '09 v4.3 installed here on El Capitan 10.11.4 and it works fine. There are some issues with hyphenation not working properly, Mail Merge panels are missing functionality, and the Share menu : Send via Mail is inoperable.
Apple will not warn us that a future OS X release will break Pages '09. They have retired it, and give it no thought. Every document that I do in Pages '09 (and Pages v5.6.1) is also exported as a Word document. If it is complicated, a PDF as well. Opening Pages '09 documents, and allowing them to be converted to Pages v5 document format will destroy your original content that is not supported in the latest Pages release.