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PDF is usable when you want to keep exact look of document. Or not allow easy modification. There is uses for it, but fiction isn't one of good ones.
Other formats allow applications to reform text better. Such as font size and line length.
EPUB and Mobi are popular formats designed for certain purpouse. And for popularity, it's often about what came at right time or was adopted by right parties. HTML is general purpose mark-up language it can do the job, but there is better options. RTF has good portability, but still it's not best or as good.
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