I've had a few strange experiences shuffling between three Macs. The problems only seem to happen when I try to open a Pages file created on the Old Mini running OSX 10.8.5 (12F45),
on the Imac running OSX 10.8.5 (12F2560). Can I get hold of a version of Pages that will run on 10.8.5 (12F2560)?
You need a copy of iWork 09 that will give you the 'old' version of Pages. However, you must have a copy somewhere to have Pages running on the 'old Mini'? So you could just install it on the iMac (assuming you have a family licence).
Otherwise: iWork came on a DVD in those days so you should be able to find one somewhere. There is then a well-hidden update package on Apple's web site that will bring you up to the last version of Pages before Apple aligned it with iOS by creating the first release of the current Pages (a completely different program).
Thanks Mick and Derek, I may give that a try, they keep offering it to me; however I've got a dim recollection that I've tried a later version of Aperture and didn't like it. I could install that OS on an external drive though and see what transpires.