Assuming that you trust the source of these files, so really do want to open them...
Have you tried, in Finder, Control-Click (or right-click) on the file, then Open With and the App Store item? This will tell you about any apps in the store that can open the files.
Otherwise, your local printshop might have indesign and be prepared to run out a PDF for you. I presume you don't feel like buying the app, though you could take up the free trial from Adobe??
Hi Tony, I trust the source as I've just paid 820 euros for them. I was a bit suspicious when I saw that the file was a mere 1.3 Gigabytes instead of the 10 GB I was expecting. They've been greatly compressed which I don't want so I've asked them nicely to send me PNGs hot from the scanner!
Eric Jervis 