Once the PDF is converted to TIFF, the digitial signature is destroyed, and all information in the signature is lost. You would need to get the information from the signature prior to converting to TIFF and "save" it somehow. The signature information will not be included in\with the TIFF file unless you could add the bits of signature information (i.e. signer name, signature status, etc) you needed to the metadata of the TIFF file, but remember, the original dig sig is still destroyed. By the way, I don't know how you could this.
Regards
Steve
