Signing PDFs has become a daily task for most DoD personnel, and getting it right — especially the distinction between a real CAC digital signature and a simple image signature — matters more than most people realize. As someone who has sent the wrong kind of signature on official documents more than once before learning the difference, I know what trips people up. Today, I will share it all with you.
CAC Digital Signatures vs. Image Signatures
A digital signature using your CAC is not the same as drawing your name on screen. It uses the certificates stored on your CAC chip to cryptographically sign the document, creating a verifiable record tied to your DoD identity. When a recipient opens the signed document, they can confirm it was signed by a specific certificate holder and that the document wasn’t altered after signing.
An image signature is a picture of your handwritten signature placed on the document. It looks like a signature. It provides no cryptographic assurance of who signed or whether the document was changed afterward.
For official DoD documents requiring authenticated signatures, always use the CAC digital signature. Image signatures are for situations where visual confirmation is sufficient and authentication isn’t required.
CAC Digital Signature in Adobe Acrobat DC
Prerequisites: CAC in reader, ActivClient or DoD middleware installed, Adobe Acrobat (not just Reader without configuration), and DoD root certificates installed on your computer.
- Open the PDF
- Click Tools, then Certificates
- Click Digitally Sign
- Draw a signature field on the document where your signature should appear
- Select your CAC certificate from the dialog — it will show your name and the DoD as issuer
- Enter your CAC PIN when prompted
- Click Sign and save
The signed PDF will show a signature block and a blue checkmark indicator confirming validity.
If certificates aren’t appearing: make sure ActivClient is running (check the system tray) and your card is properly inserted. You may need to restart Adobe after inserting the card.
Common CAC Signature Errors
No certificates available: ActivClient isn’t running or the reader isn’t recognized. Check the system tray icon first.
Signature shows invalid or unknown: The recipient’s computer doesn’t have DoD root certificates installed. This is a recipient-side issue, not a problem with your signature.
Unable to sign error: The PDF may be locked by its creator. Contact whoever sent it — they need to enable signing on their end.
Adding an Image Signature
For documents that don’t require cryptographic authentication, an image of your handwritten signature works fine.
Create the signature image: sign on white paper with a black pen, photograph or scan it, crop tightly around the signature, and save as PNG. A transparent background looks cleaner on documents — remove the white background in Preview on Mac or with a tool like remove.bg. Then in Adobe Acrobat, click Edit PDF, add the image, resize and position over the signature line.
Adobe Fill and Sign
Adobe Reader’s Fill and Sign feature lets you create a signature within the application without a physical signature image. Click Fill and Sign in the Tools panel, click the pen icon, choose to draw, type, or upload an image signature, and place it on the document. This is the fastest option for one-off signatures on documents that don’t require formal authentication.
Probably should have mentioned this option first for people who just need to sign a quick form — it works, it’s fast, and it requires no setup beyond having Adobe Reader installed.
Mac Note
Mac users can sign PDFs through Preview using the trackpad or camera signature feature — fast and convenient for non-authenticated signatures. For CAC digital signatures on Mac, you need the DoD CAC middleware plus Adobe Acrobat. Preview does not support PKI/CAC digital signatures.
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