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:
By an administrative user under Site Explorer>Forms>Data tab
By the applicant, student or end user of the form. If you want to prevent users from deleting their own forms, disable the option under Site Explorer>Forms>Edit the form in question>Click the Login tab>Unmark Allow user to delete form>Click Save.