If you have the iWork '09 DVD, it will install on Yosemite, though you will have to update it with the Apple iWork 9.3 update, which brings all of the '09 applications to their last version, and Yosemite compatibility.
Pages '09 documents contain an internal file named index.xml, and Pages v5 documents, being of an entirely different document architecture — do not. Consequently, Pages v5 documents are not directly backwards compatible with Pages '09, and if you attempt to double-click on one, or otherwise open it, Pages '09 will complain with that index.xml file is missing dialog. This is also true of Numbers '09 and Numbers v3.5.3 documents on Yosemite.
Once you open a Pages '09 document in Pages v5, and it is converted/saved, it is no longer a compatible Pages '09 document. If the Pages '09 document contained features unique to Pages '09 that Apple did not implement in Pages v5, they will be either removed, mangled, or Pages v5 may simply refuse to open the document. The only return route to Pages '09 for a Pages v5 document is via the Export to Pages '09 choice.
