How to add custom stylesheet to print pdf output?
One of the benefits mentioned on the PDF using mPDF project page is "Add custom Style Sheet to PDF". Here are some more quotes about this module (from the module's project page): 


  ... used for Conversion of HTML pages to PDF documents using mPDF PHP Library. It allows you to generate a pdf document of any node.
  
  Features
  
  
  PDF creation (www.example.com/node/nid/pdf) where nid is the node id of content to render.
  PDF generation API is provided, the content developers can use this API to generate a PDF document.
  By creating your own CSS file and/or the node.tpl.php template files, it is possible to change the look of the output page to suit your taste.
  
  
  Benefits
  
  
  Document Properties
  PDF Page settings
  Watermark Image/Text
  HTML Header &amp; Footer
  Password Protected PDF
  Add custom Style Sheet to PDF
  Enable/disable PDF generation for each Content type
  Display/ hide any fields of Content type for PDF