Being that I have frustrated no end by this ridiculous message with NO solution (that I could find) from Apple, here is what I finally stumbled on.
After a recent "re-start" I could not open some documents. Always got the "xml" message.
Tried all sorts of things. Updated to Yosemite - wouldn't run. TRIED to update Pages to 5,x (had 4.x before) - no dice. Then Yosemite finally updated. Pages ? I got the "installing" message but that NEVER ended.
After more frustration, I finally found a particular document I had had on my "startup: list that was PASSWORD protected. For whatever reason, on re-start, the password protected file didn't come up.
So, finding THAT doc in my pages list, I clicked on it and the password box came up. I entered the password and the document appeared. Once I did that the other docs that previously got the "xml" message appeared as well. And finally, I find that I DO HAVE Pages 5.x installed after all (Only Pages 4.x was showing before.
Note - for about a week or 10 days now something (automatic update ?) has changed in my MAC. When I click on say "Pages" (in the dock) my Chrome browser slides to the right instead of simply staying in the background. My menu bar also "hides" where before it stayed at the top. I cannot find any "fixes" for these.