Hi,
Your service is impressive, it does a pretty good job making the pdf from web pages.
I think the integration point would be at the layout.master file of a skin, this way its on every page of the site. How best to integrate it I'm not sure. I mean you could implement a webcontrol that inherits from <asp:Hyperlink and add your js to it, but then users would have to register the tag prefix to use your control in layout.master. I think the easiest is just instructions how to paste in either your javascript widget or a direct link like this into the layout.master file of a skin:
<a href="javascript:void(location.href='http://www.web2pdfconvert.com/?u='+escape(location.href))">Save As PDF</a>
I like this better than loading external js for your button. An img could be used in the link easy enough as well.
Best,
Joe