Originally Posted by QuietShelfLife
The short answer is that PDF has password protection baked into the format spec — every PDF reader knows how to handle it. EPUB is basically a zip of HTML files, so there's nothing like that built in. You'd need an external DRM system (Readium LCP, Apple FairPlay, Adobe ADEPT) and those all require certified reader apps to work. It's a fundamental design difference between the two formats.
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