Hi,
Even the PDF is produced from the output of a smartform. So error in pdf mode is basically a smartform error.
Please check the sy-subrc after the smartform call in the driver program in debugging mode.
If sy-subrc is not 0 the variables sy-msgv1, sy-msgv2, sy-msgv3, sy-msgv4 will have the message which will the the component of smartform which is causing the problem.
Try it and revert.
Regards,
Ashish