The real answer is what the major open source documentation projects, such as the Linux Documentation Project, do: write documents in a format (tldp uses DocBook, there are others out there) for which there are automated tools to convert it to all proper formats. TLDP example. The formats include HTML (both in multiple and single page formats - for quick loading, or ease of reading in a browser, offline, respectivly), PDF(for printing) and many other formats. They offer the various formats for download, with the multiple page ones (like HTML with images, or divided into multiple pages) as ZIP archives (and maybe also as tar.gz archives, for tradition's sake), all created automatically.

That's the real answer, but it's less easy than just hacking together a HTML or (shudder) Word doc.

/me gets off my soapbox...