Until now there has been no way to give people comment-only access to a document in Google Docs. That’s a problem because you may want feedback on a document without letting someone actually change it. According to this Google Docs blog post, Google Docs accounts will start gaining the option of sharing a document with people in such a way that all they can do is add a comment. You can even set public documents such that anyone can comment. With this change, Google Docs becomes an even more valuable collaboration tool — there’s almost no reason to mail Word documents around for comment any more. follow link
Go Back and Forth Fast in Preview
If you're reading a PDF in Apple's Preview software, and you follow a bookmark or an internal link to move around within the PDF, you can quickly return to where you were by pressing the keyboard shortcut Command-[ (that's Command-Left Bracket). Or, you can choose Go > Back.
The command works iteratively, so you can go back to just the previously viewed page or if you issue the command again, to the page before that, and so on. There's also an equivalent Go > Forward (Command-]).
Google Docs Gains Comment-Only Access Option