Does anyone else have this problem or an idea how to remedy:

Just upgraded to Mountain Lion and am getting used to most of it (though I am really sad about loss of my beloved Mosaic screensaver). A major inconvenience for a graphic designer is that if I send out a pdf proof to someone, they request changes and I go to make/replace the existing pdf in their file, it tells me I have no permission as the file is in use. I have to quit/restart mail program before I can save the corrected pdf. Seriously annoying.

Any help would be much appreciated.