Pages 5 has an oddball zipped package file format, that only Apple seems to understand and support.
All 3rd party server software such as GMail, Amazon, Google and DropBox don't understand it and can either reject it or damage it.
Once the damage is done, the internal xml structure seems to be unconnected, and except for looking at the rough hexadecimal text content, only the images are usually resuscable.
iCloud does a great job of then synching and overwriting all your good copies.
If you have tried all the usual attempts to open it, I suggest looking for a backup copy, say in Time Machine.
That's it. Well there is the commonsense approach: avoid Pages 5 at all costs.
Pages 5 is an undeclared public beta.
Peter