ePUB has the advantage of being:
1) An open standard
2) Based on well-known and well-defined formats (XHTML, CSS)
3) Capable of decent formatting, and storing metadata
4) Easy to convert from and relatively human-readable
So, even if at some point ePUB becomes obsolete, chances are you'll still be able to retrieve with minimal hassle all the text, formatting and metadata.
Of course, I bet PDF will also have a long life, but I wouldn't buy books in PDF format. PDF is good if you create it with your own preferences, for your own reader, from some source content, but as a source format itself (for future use or conversions) it has many drawbacks.
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