MilitarySmartCards

Signing PDFs & Email

Digitally sign forms with Adobe Acrobat Reader and set up S/MIME email in Outlook — the two tasks people actually need their CAC for.

Last reviewed: July 2026

Signing a PDF

Most DoD forms have signature fields that accept a CAC-backed digital signature. The free Adobe Acrobat Reader handles this on Windows and macOS.

Windows

  1. Insert your CAC, then open the PDF Windows makes your card's certificates available automatically; Acrobat Reader finds them through the Windows store. Open Reader after the card is inserted.
  2. Click the signature field On a form with a prepared field, click it, choose the digital ID that lists your name and signature certificate, and select Sign.
  3. Enter your PIN and save Acrobat requires saving the file to apply the signature — pick a new filename if you want to keep the unsigned original.

No signature field? Use All tools → Certificates → Digitally sign and drag a box where the signature should go.

macOS

Acrobat Reader on macOS can't always see the card natively. If no digital ID appears, point Reader at the system's PKCS#11 support or create the signature appearance from the Keychain-visible identity: Preferences → Signatures → Identities & Trusted Certificates. If your workflow allows it, signing on Windows is smoother; third-party tools that use the macOS keychain directly also work well.

"Certify" vs "Sign": plain Sign is what forms want. Certify locks the document against changes and confuses some workflows — use it only when a process explicitly asks for it.

Signed & encrypted email (S/MIME)

Your CAC's email certificates enable S/MIME: signing proves a message came from you unaltered; encryption makes it readable only by the recipient. Desktop Outlook is the standard tool.

  1. Confirm your email certificates match your address S/MIME only works if the address on the certificate matches the account you're sending from. If you've changed services or converted between military and civilian status, update your email address via the DoD's ID Card Office Online portal, then get updated certificates loaded onto your card there too.
  2. Open Outlook's S/MIME settings File → Options → Trust Center → Trust Center Settings → Email Security → Settings… under Encrypted email.
  3. Choose your certificates For the signing certificate, pick your CAC signature certificate; for encryption, your CAC encryption certificate. Hash algorithm SHA-256 is the safe default.
  4. Sign or encrypt a message In a new message: Options tab → Sign (or Encrypt). Outlook will prompt for your PIN via Windows when you hit Send.

Reading old encrypted email

Encrypted mail is locked to the encryption key it was sent to. When you get a new CAC, it carries a new key — old messages need your old keys, which the DoD escrows for exactly this reason. Search for your service's Automated Key Recovery Agent (ARA) process on an official .mil site to retrieve previous encryption keys, then import the recovered file into Windows.

Webmail varies. Browser-based Outlook (OWA) supports S/MIME only through a specific extension on specific browsers and configurations, and the details change. If webmail S/MIME fights you, desktop Outlook is the reliable path.

Alternatives to Adobe

Any PDF tool that can use Windows certificate stores or PKCS#11 can sign with a CAC — including several open-source options. If your workplace accepts them, they work fine; the signature format (PAdES) is standardized. Just avoid uploading DoD forms to random "sign a PDF online" websites: your document leaves your control, which is exactly what a digital signature setup exists to prevent.