[Quoth]

Edge can't read other than HTML online:
1) The current version doesn't have an ereader now
2) When it did, it was fake Online, like a PDF plugin. Actual ebooks and PDFs, even if viewed in a browser are actually being downloaded on a temporary basis.

Actual eBooks or PDF can only be really read by download (though it might not be obvious), anything else is really HTML.

So if it's a real ebook or PDF, then download it and read with any suitable application to taste. No need to stay connected.

Also nothing reads Excel, Word etc "Online". Google Docs and similar allow a download export of a real document, the actual online version is being done by HTML, javascript, CSS etc from an interactive server.