I had to force-quit Preview (it became unresponsive after trying to open a 200Mb animated gif), but after re-starting it, some files that were open before no longer open. Instead an error box pops up saying that
The file “file.pdf” couldn’t be opened because you don’t have permission to view it.
To view or change permissions, select the item in the Finder and choose File > Get Info.
(how can I generate an image of that error box?).
However, the file permissions are okay, finder generated the usual preview, and acroread can open the file. I suspect that Preview has generated some lock for this file somewhere, but I'm not sure where and how. OSX 10.9.5. (copying the file in a terminal to another one and trying to open that didn't help, as did changing permissions to read-write for everyone).