Attach file to comment. Typically the client add in quick sketches, reference images, new logo files and so on and it would be super helpful if they could attach those files to the comment in review mode to avoid having to still use e-mail or similar services to get quick assets between us and them.

Deeper zoom function: The pdf can only be zoomed in to 100% of the artboard setting, so some details can't be seen. Infinite zoom would be ideal.