Element <SIGNATURE>
Contents
SIGNATURE adds a digital signature: this is a security feature that 
EscapeE can add to make PDF documents tamper-proof, and enables signatures to be validated. See Security options for PDF export.
Digital signatures need the certificate name and a serial number. You can also have a description e.g.
<SIGNATURE CERTIFICATE="DGF LLC" SERIAL="7C 56 05 3F 88 42 13 88 45 95 4A 2B 03 C7 56 47 " DESCRIPTION="Test file">
[digital signature encoded in base 64]
</SIGNATURE>
IDF wizard helps with setting these up: check the "Sign" box when creating the IDF file.