I am in the midst of finding out how to encrypt work shared digitally, especially for the purpose of getting beta readers to critique our work and to get #bookreviewers and #booktubers review our books. Author Review Copy (ARC) is what we call the books sent to readers in exchange for a review, but when it is sent in PDF form, it is just a matter of time before the whole book gets copied and sold elsewhere.
A simple google search moments ago helped me discover this website which I believe has a lot more other useful tips for authors and I would highly encourage you to read it. Also let me know in the comment below if you have actually used any softwares that help you safely sell encrypted e-books or if any of these precaution measures helped you in the past?
Ebook Theft: 12 Things You Can Do to Stop Someone from Copying and Selling Your Ebook