How to export Drupal page as PDF?
You can use the Print module 


  This module allows you to generate the following printer-friendly
  versions of any node:
  
  Printer-friendly version (webpage format) (at
  www.example.com/print/nid) PDF version (at
  www.example.com/printpdf/nid) Send by email (at
  www.example.com/printmail/nid) where nid is the node id of content to
  render.


And your choice of one of the 4 PDF libraries it supports. It does have very high memory requirements when generating PDF as previous comments have noted. It is well documented and the PDF output is fully themeable.