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Dabcar - I faced this issue a few years ago, distributing documents to a closed user group who had different devices, some users had more than one e.g. a Kindle and an iPad.
About 80% of the documents were uploaded as EPUB, KF8 and MOBI, and 20% as PDF, the latter had lots of tables, infographs etc. Some documents (urgent ones) were only made available on a secure website as web pages.
Publishing on Amazon/Kobo was out of the question, they were 'secrets'.
Eventually the whole kit and caboodle was turned into an ios/android game app using a x-platform development tool… Unify… Unity… something like that.
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Tex2002ans FWIW: the content of EPUBs are searchable on Windows if you install an EPUB IFilter ==>>
Content Search EPUB. And IIRC the DOCFetcher tool can also search content of EPUBs - on Windows, Linux and MacOS.
BR