is the form page named thankyou_pdf.php? or is thankyou_pdf.php the second page?
I ask this because the forms action is set to post to thankyou_pdf.php:
<form action="thankyou_pdf.php" method="post">
this is an important detail to note because all for processing must be performed on the forms action page.
you have the set session value code on the same page as the form, this will only work if the form is set to post to the same page that contains the form.
If the form is set to post to another page, the set session value code needs to be on that other page.
another important detail to note is the trigger. you have selected to use the Currant Page Submit trigger.
this trigger cannot be used if the form is set to post to another page.
Finally, make sure that all of your form elements have a values attribute, your text field is missing this:
<input name="sessiontest" type="text" />
some browsers will not post a form elements value if there is no value attribute:
<input name="sessiontest" type="text" value="" />