In my own experience, switching from a double-click/tap on the Pages document icon, and using the File ▸ {Open, Open Recent} menu items seems to get past this random Pages v5 document amnesia.
Since Apple chose to retain the same .pages extension for Pages '08, Pages '09, and Pages v5, it is difficult to know what application version created the document. Try to open a Pages '08 document in Pages v5 and it will tell you it cannot open it, and go use Pages '09. Dare to open a Pages v5 document in an earlier Pages and you learn about index.xml files that are missing. Attempt to open a default Pages v5.5.3 single-file format document in Pages v5.2.2 on Mavericks and it blows up because it expects a package file format. Apple has lost their way.
MS Word is a business/professional grade application that in capable hands, shows the world what omissions and poor software design went into Pages v5.