I am not sure if I am alone in experiencing severe problems with Mac's Preview and file permissions since about 6 months. The files which you yourself have created on the same machine start giving problems with permissions. This happened with Mac's own Pages too. Despite making sure that the file has full edit permission for everyone, Mac's softwares are randomly generating file permission errors. I hope Mac's reputation for user friendliness remains intact for long despite such pesky file permission problems.