Can not open old PDF files on some desktops

I have a large number of seven year old pdf files on a windows 7 desktop. I can not open them on this desktop. I also can not open them on a different, windows 7 desktop. However, they will open on a different windows 10 desktop.

One odd, perhaps clue, is that the folder they are in says it is read only, and, when I attempt to remove that attribute and apply it then the attribute does not change.

I sometimes but not always get error 109. I routinely get a message saying the file may be corrupt, etc., but because it opens on a different machine know that is not the case

Suggestions?