You don't need an app - Safari can do this for you.
You need a 'bookmarklet' - check that Javascript is turned on in your Safari 'Settings'
Paste the following as a bookmark
javascript

df_url=location.href;location.href='http://pdfmyurl.com?url='+escape(pdf_url)
Now, when you want to convert a web-page to PDF, display on Safari the web page you want to convert and then select the above bookmark. It will be converted to PDF in Safari and then you can save it to iBooks or some other app that can read PDF.
Tim
PS posting puts a space in one of the http which you'll need to edit out