HTML to PDF and application level security

Hi,

 I would appreciate it if someone could help me with this one as I'm beginning to feel like i'm hitting my head against a wall.

 I'm trying to use expertPDF (www.html-to-pdf.net) to pass a url in to output a PDF document, this works without any problems until I apply application level security, I have a login page and when I pass any url within my application it outputs the login page as the PDF.
 I have tried contacting the company themselves and looking through the documentation the suggestion is to use the following:
 (2) get the web page HTML code using the Server.Execute(Url) method. The Server.Execute method is executed in your application session so all the session data and existing authentication should be valid.

When getting the HTML string with the Server.Execute(Url) method the resulted HTML code should reference the external CSS, images a ...