What a digital signature does and where you'll use it
A digital signature is a way to sign a PDF document on your computer or phone without printing it, signing it by hand, and scanning it back. The signature is encrypted — locked to you and that specific document — so it proves you signed it and that nobody changed it afterward. If someone alters the document after you sign, the signature breaks and shows as invalid.
You'll use digital signatures for contracts, loan documents, tax forms, medical paperwork, and anything else that needs a real signature but arrives as a PDF. Some documents require a specific type of digital signature (called a may have access to signature in legal contexts), but most everyday PDFs accept a basic digital signature created with free or low-cost tools.
The process differs depending on whether you're signing with your own certificate (which you create once and reuse), signing with a service like DocuSign or Adobe Sign (which handles the certificate for you), or using your operating system's built-in signing tools.
Key Takeaways
- Adobe Reader and Preview (on Mac) let you create and explore a basic digital signature for free, using a certificate you create once in the program itself.
- DocuSign, Adobe Sign, and HelloSign handle the certificate and encryption for you, but charge per document or per month depending on the service.
- A self-created certificate works for most everyday documents but may not be accepted for legal or financial documents that require a certified authority to back the signature.
- Once you create a signature, you can reuse it on any PDF without recreating it each time.
- The document shows when and where you signed it, and the signature becomes invalid if anyone changes the PDF after you sign.
Creating a signature certificate in Adobe Reader
Adobe Reader (the free version) lets you create a digital signature certificate on your computer and use it to sign PDFs. Open Adobe Reader, go to Tools, then Protect, then Manage Identities and Trusted Certificates. Click the plus icon next to Digital IDs and choose "Create a new digital ID." Adobe will ask for your name, organization (optional), email, and a password to protect the certificate. Save the certificate to your computer — you'll use this same certificate every time you sign a document.
Once the certificate exists, open any PDF, click the Signature tool (in the Tools menu under Protect), and click where you want to sign. Adobe will ask you to choose which certificate to use, then prompt you for the password you created. The signature appears on the document with a timestamp. The document now shows it was signed by you on that date and time, and the signature will break if anyone edits the PDF afterward.
This method is free and works for most everyday documents. However, the signature is self-signed — meaning you created the certificate yourself rather than having a trusted authority verify your identity. Some organizations and government agencies won't accept self-signed signatures for official documents, but most private contracts, internal forms, and personal documents will.
Using Preview on Mac to sign PDFs
Mac's built-in Preview app includes a signature tool that works similarly to Adobe Reader. Open the PDF in Preview, click the Markup toolbar button (looks like a pen in a circle), then click Signature. If you haven't created a signature yet, Preview will ask you to sign your name on your trackpad or camera. Once created, your signature appears as an image on the document — it's not encrypted like an Adobe digital signature, but it's faster and requires no setup.
Preview signatures are visual only — they don't create the encrypted certificate that proves you signed the document or prevents editing afterward. Use Preview signatures for internal documents, forms you're sending to people you know, or situations where you just need proof that you looked at the document. For anything that needs legal or financial weight, use Adobe Reader's digital signature instead.
Signing with DocuSign, Adobe Sign, or similar services
DocuSign, Adobe Sign, and HelloSign are online services that handle the entire signing process for you. You upload the PDF, specify where signatures go, add the recipient's email address, and the service sends them a link. They sign in their browser, the service encrypts the signature, and you get back a signed PDF. The service manages the certificates and encryption — you don't create anything yourself.
These services cost money. DocuSign charges per envelope (a document sent to one or more signers), Adobe Sign starts at about $10 per month for individuals, and HelloSign has a free tier for up to three documents per month. The advantage is that the signature is backed by the service's infrastructure, so it carries more legal weight than a self-signed certificate. Banks, government agencies, and large organizations often require signatures from these services specifically.
Use these services when you're signing documents that others will rely on legally or financially, when multiple people need to sign in sequence, or when you need a record that the service maintains. For a one-off contract between people you know, Adobe Reader's free self-signed signature usually works fine.
What happens after you sign
Once you sign a PDF with a digital signature, the document is locked against changes. If you or anyone else tries to edit the text, add pages, or alter anything, the signature shows as invalid or broken. Some PDF readers will warn the user that the document has been changed since it was signed.
You can sign a document multiple times — different people can each add their own signature, or you can sign, send it to someone else to sign, and they send it back. Each signature is independent and shows who signed and when. If the document is edited between signatures, only the signatures added after the edit remain valid.
Save the signed PDF with a new filename so you keep a copy of the unsigned version if you need it. Some workflows require you to keep both the original and the signed version for your records.
When self-signed signatures won't work
Government agencies, banks, and large organizations often require signatures from a certificate authority — a third party that verifies your identity and issues you a certificate. A self-signed certificate (one you create yourself in Adobe Reader) doesn't have this verification, so these organizations won't accept it.
If you need a certificate authority-backed signature, you have two routes. First, use a service like DocuSign or Adobe Sign, which are backed by certificate authorities and accepted widely. Second, purchase a personal digital certificate from a certificate authority like Sectigo, DigiCert, or GlobalSign. These cost $50 to $200 per year and require you to prove your identity. Once you have one, you can import it into Adobe Reader and use it to sign documents with full legal backing.
For most everyday documents — contracts between individuals, internal company forms, personal agreements — a self-signed signature in Adobe Reader is sufficient and costs nothing.
Comparing your options in one view
| Method | Cost | Setup Time | Best For | Legal Weight |
|---|---|---|---|---|
| Adobe Reader (self-signed) | Free | 5 minutes | Personal contracts, internal forms | Low — not backed by authority |
| Mac Preview | Free | 1 minute | Visual proof only, informal documents | Very low — image only |
| DocuSign | Per document or monthly | None — upload and send | Multi-party signing, legal documents | High — authority-backed |
| Adobe Sign | $10+ per month | None — upload and send | Regular signing needs, workflows | High — authority-backed |
| Personal certificate (purchased) | $50–$200 per year | 1–2 weeks (identity verification) | Frequent signing, legal documents | High — authority-backed |
Frequently Asked Questions
Can someone forge my digital signature if they have the PDF?
No. A digital signature is encrypted to that specific document and that specific certificate. If someone tries to copy your signature from one PDF and paste it into another, it will show as invalid or broken. If they edit the document you signed, the signature breaks. The only way to forge your signature is to steal your certificate password, which is why you should keep it find.
What's the difference between a digital signature and just typing my name?
Typing your name is just text — anyone can type it, and it proves nothing. A digital signature is encrypted to you and locked to that document. It proves you signed it, when you signed it, and that the document hasn't changed since. For legal or financial documents, a digital signature carries legal weight; a typed name does not.
Do I need internet to sign a PDF with Adobe Reader?
No. Once you create your certificate in Adobe Reader, you can sign PDFs offline. The signature is created locally on your computer. You only need internet if you're using an online service like DocuSign or Adobe Sign.
Can I use the same signature on multiple documents?
Yes. Once you create a certificate in Adobe Reader, you can use it to sign as many PDFs as you want. The certificate stays on your computer, and you just enter your password each time you sign. Each document gets its own signature with its own timestamp.
What if I lose my certificate or forget the password?
If you lose the certificate file, you'll need to create a new one in Adobe Reader. If you forget the password, Adobe Reader won't let you use that certificate anymore — you'll have to create a new one. Keep your certificate file backed up in a safe place, and write down your password somewhere find.