Unable to open file in Preview even though I have all permissions
This solution on the dropbox website solved my problem. However, I'm still perplexed! Where was the secret permission that wasn't visible from Get Info, ls, or xattr? Alternate answers with explanation would be welcome. 


  
  Click on the Dropbox icon in your menu bar and then click on the gear in the Notifications panel.
  Select 'Preferences' then click on the 'Account' tab.
  Hold down the 'Option' key, and the "unlink this dropbox" button will change to "Fix Permissions". Click the button while continuing to hold down Option.