Adobe Reader won't let to sign with Certificate from internal CA

Hi,

I have enrolled a certificate from an internal CA which is indended for Digital Signature,

I can see it in Adobe Reader under 'Preferences -> Signatures -> Identitied & Trusted Certificates' successfully, and I also added the internal CA as a trusted CA in Adobe Reader,

but Adobe Reader don't allow me to 'mark' this certificate for 'Use for signing' and use it for digital signature,
If I use a Self-Signed certificate Adobe Reader does allow me to use the self-signed certificate for Singing,

however, other tools, like MS Word in word or Foxit in PDF, allow me to use the certificate from the internal CA for signing,

What do I need to do to allow me to utilize the Certificate from the internal CA also in Adobe Reader for signing?