<?php
   //check if the session variable exists
   session_start();
   if(isset($_SESSION["pdf"])){
     //it exists : do your thing

   }else{
      //set the session variable
      $_SESSION["pdf"] = "mypdf_file.pdf";
   }
?>

session_start();
 if( isset($_SESSION['pdf_name']) ){
       //session variable set. Resume the pdf viewing
 }
 else{
     $_SESSION['pdf_name'] = <pdf name>
     $_SESSION['page'] = <page no.>
 }