“Preview” is damaged and can’t be opened. You should move it to the Trash.
This was the rather surprising error message that I’ve been getting when I try to open a PDF from the Finder since I upgraded to OS X Yosemite. It’s bad enough when you get an error message, but one suggesting that you delete a frequently used app is inconvenient to say the least!
Since I found it, I’ve been using a workaround: start Preview.app and open the file manually.
But there’s a better answer. Apparently it just has a duff “quarantine” attribute set. To get rid of it and get things back to normal simply do the following in a Terminal window:
/Users/stephend $ cd /Applications /Applications $ xattr -l Preview.app com.apple.quarantine: 0006;53563f06;Acorn; /Applications $
If you see the same thing, proceed to the next step:
/Applications $ sudo xattr -d com.apple.quarantine Preview.app Password:****
And that’s all. Now opening a PDF from the Finder should work. It’s bizarre.
I raised Radar 19384558 with Apple. Please dupe if you see the same thing. Thanks to Gus Mueller (developer of Acorn) for the support.
Update: It seems that a couple of other apps also have the same, incorrect attributes:
/Applications $ xattr -l *.app Utilities/*.app | grep Acorn    Mail.app: com.apple.quarantine: 0006;53563f06;Acorn; iPhoto.app: com.apple.quarantine: 0006;53563f06;Acorn;
I have also removed those tags, though I’ve not seen any problems with them so far.