Example: The url you want to render has a query string in it like
http://www.google.com/search?hl=de&source=hp&q=qualidator&meta=&aq=f&oq=

If you want to pass this url to html2pdf you have to make sure you url encode this url before adding it to your rendering path and query.

The encoded URL looks like this:
http%3A%2F%2Fwww.google.com%2Fsearch%3Fhl%3Dde%26source%3Dhp%26q%3Dqualidator%26meta%3D%26aq%3Df%26oq%3D

Use a URL encode like http://www.albionresearch.com/misc/urlencode.php for that.

The final URL to call the rendering service then is:
http://convert.html2pdf.seven49.net/?url_to_render=http%3A%2F%2Fwww.google.com%2Fsearch%3Fhl%3Dde%26source%3Dhp%26q%3Dqualidator%26meta%3D%26aq%3Df%26oq%3D

If you want to add more parameters this is no problem. Example:
http://convert.html2pdf.seven49.net/?url_to_render=http%3A%2F%2Fwww.google.com%2Fsearch%3Fhl%3Dde%26source%3Dhp%26q%3Dqualidator%26meta%3D%26aq%3Df%26oq%3D&FooterText=Your+footer+text+goes+here