Problem sovled by changing the server port number
from
bundle exec merb -a thin #default port 4000
to
bundle exec merb -a thin -p 3000
and using the gem "wkhtmltopdf-binary"
Explanation
can sometimes find that the single threaded web server we use in development can cause a race condition. the browser is tieing up the one thread available, and begins a new web request to create the pdf files html, which causes a 2nd web request. Since your only web server thread is already in use, you can find this to hang.
To get around this, either run more threads. OR use another port for internal access, and have be the internal URL