Reverting back to Mavericks from El Capitan
There is no real way to easily downgrade any version of Mac OS. You have two choices here.


If you made a backup image of your HD before you upgraded, back up the files that have changed since then, boot from that image and restore it to your boot drive.
Back up your current HD, Wipe it and reinstall the previous OS. Then reinstall your apps and restore your data from the backup.


Honestly sounds more like an issue with Pages than with the OS, but since I don't use Pages I have nothing useful to suggest other than removing and reinstalling the app. Perhaps if you edited your question asking for help with Pages on El Cap....