Diagnosis
The Print Preview function can be run only if you haveimplemented the method DISPLAY_PDF_DOCUMENT in Business Add-In(BAdI) USMD_UI_EVENT and that method returns the PDF document tobe displayed. However, since you either have not implemented the BAdI orDISPLAY_PDF_DOCUMENT does not return the PDF document, the printpreview function cannot be executed.

Procedure
Implement the method DISPLAY_PDF_DOCUMENT in BAdIUSMD_UI_EVENT.