Some epubs have a map of correspondences, for each page number of the paper book there is a mark in the epub, <a id="page_23"></a>. And in that case there is often a file mapping such anchors with their page number.
Obviously, this feature depends of the good will of the editor that created the epub, this is not a generic solution.
For the epubs without this feature, it's easy to go from the paper toward the epub, a term of phrase can be searched. The contrary is clearly more problematic.
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