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Lumin PDF lets you annotate PDF docs online, using Mac or Windows or from an iPhone or Android Phone. You can also use Lumin PDF as an Google Drive extension and work comfortably with Google Docs, Google Sheets and your PDF files. 
Unlike a long email with comments like ‘on page 23, top left, at the bottom of the picture’ they can simply place a little comment bubble in that location and add a few notes. They might use these comments to explain particular design choices, post little queries to you or highlight something that needs attention
Once a note is added, the selected text will be highlighted, and a comment icon will appear to indicate the comment. In order to preserve those comments, you will need to save the changes to the PDF document