A server module is required that can be called from your site's scripts. Going by the categories you've posted the question into, this is an ASP.NET site on IIS. So you're looking for a html to pdf module compatible with IIS and adds the functionality to ASP.NET
I believe there's little choice for free or opensource dll's to download and install. This is beside the point though. Before you can even go shopping for a pdf converter, you have to contact your website host and ask if you can have 3rd party modules installed on the server.
Asking google, 'asp.net html to pdf', yeilded quite a few converters: -
http://www.websupergoo.com/abcpdf-1.htmhttp://www.codeproject.com/KB/aspnet/HTML2PDF.aspxhttp://www.duodimension.com/html_pdf_asp.net/component_html_pdf.aspxhttp://www.asppdf.com/..to name a few.