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You have to use an Adobe app if you want the comments to "stick." Here is the same marketing copy, but with the important bits emphasized.
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What the above marketing copy doesn't say is that you can't use the Dropbox app or the Dropbox website to make your PDF comments.
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Access the PDFs in your Dropbox from within Acrobat DC or Acrobat Reader - on your phone or tablet, or on your computer.
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