Mikes suggestion could work for PDFs. As I understand it, Sparse files are supposed to be a very efficient way od compressing, but I don't get it. It obviously works for PDFs, as you are succesfully opening them. But I believe it strips the visual content of Avis, leaving only the headers. Ihope I am wrong, but I doubt you will be able to get any use from them.
But, you have no idea what program made the files? I suspect an online game? Perhaps you could move them all to a temporary folder and see if there is anything not working on your computer after a few days.