You have 2 versions of Pages on your Mac.
Pages 5.2 is in your Applications folder.
Pages '09/'08 is in your Applications/iWork folder.
You are alternately opening the wrong versions.
Pages '09/'08 can not open Pages 5 files and you will get the warning that you need a newer version.
Pages 5.2 can open Pages '09 files but may damage/alter them. It can not open Pages '08 files at all.
Older versions of Pages 5 can not open files from later versions of Pages 5.
Once opened and saved in Pages 5 the Pages '09 files can not be opened in Pages '09.
Anything that is saved to iCloud and opened in a newer version of Pages is also converted to Pages 5 files.
All Pages files no matter what version and incompatibility have the same extension .pages.
Pages 5 files are now only compatible with themselves on a very restricted set of hardware, software and Operating Systems and will not transfer correctly on any other server software than iCloud.
Apple has removed almost 100 features from Pages 5 and added many bugs: