Yes, you can. There are a number of PDF editors available. For example, in Preview on the Mac, your editors & reviewers can use the "Annotate -> Add Note" feature to add notes to your Leanpub-generated PDF.
Interested in creating your first Leanpub book? Here are some quick tutorials for our most popular writing modes: http://help.leanpub.com/en/articles/3088382-quick-walkthroughs-for-getting-started-on-a-leanpub-book
If you have any questions or thoughts on writing and self-publishing with Leanpub, please join our global community of authors in our Authors Forum here!
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