Digitally signing a pdf was added to ColdFusion 11. Seriously given the security and other issues with old versions, you seriously should look to upgrade your copy of CF. If your company cannot afford Adobe CF there are open source alternatives.
That said, here is a brief discussion of digital signing a pdf with coldfusion:
https://helpx.adobe.com/coldfusion/cfml-reference/coldfusion-tags/tags-p-q/cfpdf.html#cfpdf-Digitallysigningthedocuments
Creating a pdf signature is relatively easy. In essence you need a digital certificate (I've used both commercial and self signed ones - they both work the same), with information regarding the path to the certificate,
<cfpdf action="sign"
source="#pdfPath#" <!--- path to pdf C:\pdfSourcePath --->
destination="#pdfPath#"
keystore="#keystore#" <!---path to the certificate --->
keystorepassword="#keystorepassword#" <!--- cert's password --->
password="pdfPassword" <!--- password to the pdf if used --->
pages="123" <!--- page for the signature I use the last page --->
height="25" <!--- signature height with an positioning. --->
width="300"
position="68,80"
overwrite="true"
author="true" />
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