Not vaadin-the-framework, but vaadin-the-project. How is the book of vaadin constructed? How are the HTML page and the PDF kept in sync?
The reason I ask is that I like both the PDF and the HTML version and I’d love to do something similar for my product. If neat software solution is the answer I’ll be gushing for days 
The
source code
for the book is in DocBook format. It is compiled to HTML for the website and to FO for the PDF and print editions. The FO is processed into PDF using the commercial XEP FO processor. See the
build script
for details on the build process.
There are
DocBook configuration files
with various formatting customizations for HTML, Letter-size PDF, and pocket-book size PDF separately.
Guess you also have tried the Apache FOP and other open processors, what’s the biggest issues with them? Why aren’t they used?
When selecting the toolchain, Apache FOP would have been preferred as it is free. Unfortunately it did not work in practice. If I remember correctly, one of the problems was related to embedding vector graphics and there were bunch of layouting issues.
It might be that FOP has matured since 2007 when we selected XEP - if anyone is willing to try, please report back here 