Why use MS Office for that? I've built a file sharing site that uses LibreOffice amongst others to convert files to mobile and tablet friendly previews.

Best part of LibreOffice?

1) It's fast

2) You can call it via a simple command to convert a file to another format. So easy as 1 2 3 to integrate in your code. Convert to HTML, PDF, whatever, you name it :)

3) It's free

Also, just check out pandoc. You'll love it. Abiword works too.

#pandoc -o output.html input.txt

#abiword --to=doc filename.odt

I'll be opening an API soon :)

It is not illegal but violates the license

Can you expand more on this ?

Probably meaning it is not illegal in the criminal sense but violates the terms of the license (which is a civil/contract law issue).