Q: how to open new Pages file format

You should have both versions of iWork. In your Applications folder there is a folder called iWork '09 with your older apps.

And then loose, on the top level of your Applications folder is Numbers, Pages and Keynote.

For any files that you've already saved using Pages version 5 you can go to File menu > Export to > Pages '09 in order to create a copy that you can open on your older Mac or with the Pages '09 that still should be on your newer Mac.

All explained in http://support.apple.com/kb/HT6049, which discusses the new iWorks stuff (dumbed down to mimic it's iOS counterparts) and the previously full-featured iWork '09 apps.