Form asking to refresh when I hit the back button

Not sure if this is a PHP question but I've traced the problem to a PHP statement. I have a form that when submitted, displays a PDF document in the browser (using a PDF library). When I click on the back button, it should redisplay my form but instead, displays a warning that data has expired and I need to refresh. Now this never happened until recently so I look at recent changes and traced the problem to one single PHP statement at the top of my file: session_start(); I am using sessions to pass values between pages and so added the statement to my page so that I could check the value of a variable. When I comment out the above statement, I am no longer prompted to refresh. Question: How can I have my cake and eat it to? Thanks, Don