Pages v5 uses a new document architecture that is incompatible with earlier releases of Pages. Still has the .pages file extension to further confuse you. If you have Pages ’09 running, and attempt to open a v5 document (e.g. double-click), you will get the following dialog. What it really means in English is that Pages '09 cannot open Pages v5 documents, and that you should upgrade to Pages v5. Since you already have done so, or you wouldn't get this message, the dialog is nonsense. There is no newer Pages ’09 version than v4.3, and it is no longer available from the OS X App Store.
Pages v5.2.2 will periodically lose its mind, and claim that it cannot open a document created by itself. You can double-click on it until you run out of expletives, or use a right-button click and open with Pages (default) (5.2.2). This is a horrendous application.
Pages v5.2.2 will quietly open Pages ’09 documents, and because it has 110 fewer features/functionalities than Pages ’09, it will also filter out what it does not support, and then auto-save the contents. Having a backup of your Pages ’09 documents is crucial with this behavior.
I removed any Pages icon from my Dock. I then dragged each Pages application icon to my Dock, so that they are adjacent. With both installed, extra precaution is necessary, as any double-click on a document, will attempt to open it in Pages v5 first. Pages ’09 documents must be opened via that application's document chooser, or via a right-click, open with Pages (4.3) menu item.
As I have posted previously, I created two new Finder tag items. A blue one that I named Pages ’09, and a yellow one that I named sh*t Pages v5. I place these tags on the respective Pages documents to avoid a train wreck, and to aid Finder searches (e.g. tag: Pages '09.)
