This error happens if the Data for the form has been deleted.  The PDF will not be displayed and the error will occur. 

The form may have been deleted one of two ways:
A) By an administrative user in Site Explorer> Forms> Data tab
B) By the applicant, student or end user of the form. If you want to prevent users from deleting their own forms:
     1) Navigate to Site Explorer>Forms
     2) Click the pencil icon to edit the form
     3) Select the Login tab
     4) Unmark "Allow user to delete form" checkbox
     5) Click Save