How to Digitally Sign a PDF (Certificate or Not)

Most of the time, "digitally sign this PDF" means signing on a screen instead of on paper: you draw or type your signature, put it on the signature line and send the file back. Sometimes it means a certificate-based digital signature, which uses a digital ID to show who signed and whether the file changed afterwards. The tools, the steps and the cost are different, so take 30 seconds to check which one you need.

Which one do you need?

The request mentions "digital ID", "certificate", "certified", "PAdES" or "qualified". You need a certificate-based signature. Skip to Part 2.
The PDF has a signature field that asks for a digital ID when you click it in Acrobat Reader. Many fillable forms have one even when a normal signature is fine, so check the request or ask the sender before you go to Part 2.
The request just says "please sign and return" or "sign it digitally" with no other detail. Part 1 is usually what they want: your signature on the page. You can add it in SigPDF, right in your browser (free to try, pay to download). Part 1 below shows the steps.
You are not sure. Ask the sender: "Do you need a certificate-based digital signature, or is my signature on the PDF enough?" One short email can save you from signing twice.

Part 1: Put your signature on a PDF digitally

This is an electronic signature: your signature, visible on the page, as on paper. Most contracts, leases, forms and offer letters ask for this. SigPDF does it in your browser, and your PDF is not uploaded to a server.

Open your PDF. Go to SigPDF and drop the file on "Drop your PDF here", or click to choose it.
Create your signature. In "Create Your Signature", pick Draw to sign with a mouse, trackpad or finger, or Type to write your name in a handwriting-style font. Then click "Use This Signature".
Place it. Click the page where the signature belongs. Drag it onto the line and resize it to fit.
Add the rest. Use Add Initials, Add Date, Add Text or Add Photo for anything else the document asks for. Add Photo also takes a scan or photo of your ink signature (PNG or JPEG).
Download. Click "Confirm & Download Signed PDF", save the file and send it back the way the sender asked.

Free to try. Downloading the signed PDF needs a plan: €2.99 a month or €24.99 a year, cancel anytime.

More detail on each part:

Drawn, typed and image signatures, with tips for each: add a signature to a PDF.
Whether this kind of signature counts for your document: electronically sign a PDF.
PDFs with fillable fields, such as applications and tax forms: fill and sign a PDF.

The result is a PDF with your signature written into the page. It carries no certificate, so Acrobat does not list it in its signature panel. For most documents that is fine. If the sender wants a certificate, read Part 2.

Part 2: Add a certificate-based digital signature

What it is

A certificate-based digital signature uses a digital ID: a private key that only you hold, plus a certificate with your name on it. When you sign, your PDF app uses the key to sign a fingerprint of the file and stores the result, with your certificate, inside the PDF. A reader such as Adobe Acrobat Reader can then check whether it trusts the certificate and whether the file changed after signing. The signature can also show as a box on the page, but the proof is in the file, not in the picture. Our guide to electronic signature vs digital signature explains the difference in more depth.

SigPDF does not create certificate-based signatures. If you need one, use Acrobat Reader as shown below, another app that supports certificates, or the signing link the sender gave you.

How to do it in Adobe Acrobat Reader

These steps are for Acrobat Reader on a Windows or Mac computer; the Acrobat Reader phone app cannot add certificate signatures.

Adobe Acrobat Reader is free, and Adobe says both Acrobat and Reader can sign PDFs with a certificate ID. You need a digital ID first (see the next section). In the current Acrobat interface:

Open the PDF and select All tools > Use a certificate.
Select Digitally sign. Click OK on the message with signing instructions.
Drag a rectangle where the signature should appear.
In the Sign with a Digital ID dialog, choose your ID and select Continue. No ID yet? Select Configure New Digital ID and follow the prompts.
Check the signature details and enter your digital ID password. Select Lock document after signing only if nobody else needs to sign after you.
Select Sign, pick where to save the signed file, and select Save. Saving it under a new name keeps your unsigned copy.

Menu names change between versions. If yours look different, search Adobe's help for "Add digital signatures".

Where a digital ID comes from

Adobe lists three ways to get one. The details vary by country and provider.

Your organization. Many employers, universities and public bodies give staff a digital ID. Ask the IT team.
A certificate authority or trust service provider. These companies check your identity and issue a signing certificate, as a file, on a USB token or through a remote signing service. Most charge for this.
A self-signed ID you create in Acrobat. It costs nothing and takes a minute: Menu (Windows) or Acrobat (macOS) > Preferences > Signatures, select More under Identities & Trusted Certificates, then Digital IDs, the add digital ID icon, and "A new digital ID I want to create now". Nobody checks your identity, so the recipient's software will usually not trust it.

In some countries, mostly in Europe, a national ID card or eID app can also sign documents; check your government's eID website for what it supports. The guide to simple, advanced and qualified signatures covers which certificate each level needs and how to get one.

Common problems

"Signature validity is unknown"

When you open a signed PDF, Acrobat checks whether the signer's certificate, or a certificate above it, is in its list of trusted certificates. A self-signed ID is not on that list unless the recipient adds it by hand. So the recipient may see a message such as "Signature validity is unknown" or "At least one signature has problems", even though nothing in the file changed.

It means Acrobat cannot confirm who you are, not that the file was tampered with. For a signature that checks out without extra steps, use an ID from a provider on the Adobe Approved Trust List (or, in the EU, a qualified provider on the EU Trusted List), which Acrobat and Reader trust by default. An ID from your employer checks out the same way only if it comes from such a provider or the recipient's software trusts your organization's certificates.

You need to add something after the certificate signature

Adobe's help says that signing a PDF with a digital ID locks it against further changes, and Acrobat tracks what changes between signed versions. So finish the page first: your drawn or typed signature, initials, dates, text and images. Apply the certificate signature last.

Need both a handwritten-style signature and a certificate? Sign in SigPDF first, download the file, then add the certificate signature in Acrobat Reader. Keep an unsigned copy. If you are the only signer and did not select Lock document after signing, Adobe says you can right-click your signature, choose Clear Signature, make the change and sign again.

The PDF already has someone else's certificate signature

Check the Signatures panel in Acrobat Reader on a computer. If it lists a certificate signature from someone else, such as your landlord or employer, do not sign that file in SigPDF or any other editor that rewrites the PDF. SigPDF saves a new copy of the whole file, so their signature will most likely show as changed or invalid. Ask the sender how they want you to sign. Often that means signing in Acrobat Reader instead.

The recipient rejects your signed PDF

Sometimes a sender returns a PDF you signed in Part 1 because they needed a certificate signature; their software may check the signature panel, not the page. A drawn or typed signature cannot be turned into a certificate signature later. Ask exactly what they need, then sign again with a digital ID.

Is a digital or electronic signature legally valid?

In the US, the ESIGN Act and UETA (adopted by almost every state) say a signature in most business and consumer transactions may not be denied legal effect just because it is electronic. That covers drawn and certificate-based signatures alike. Some documents are excluded, such as wills and some court documents.

In the EU, eIDAS says no electronic signature may be denied legal effect just because it is electronic, and it guarantees that a qualified electronic signature has the same legal effect as a handwritten one in every EU country. What other signatures count for is left to national law.

In the UK, the Electronic Communications Act 2000 lets electronic signatures be used as evidence. Whether your document needs more depends on the law that applies and on the other side, so see are electronic signatures legally binding and simple, advanced and qualified signatures.

Frequently asked questions

Is a drawn signature on a PDF a digital signature?

Not in the technical sense. It is an electronic signature. Many people still call it a "digital signature", and when they ask you to "sign digitally" that is often all they mean.

Can I digitally sign a PDF for free?

Partly. Acrobat Reader and a self-signed ID cost nothing, but recipients will usually see that signature as untrusted. A trusted ID normally comes from a paid provider, or from your organization if the recipient's software trusts its certificates. SigPDF is free to try for a signature on the page; downloading the signed PDF needs a plan.

Does SigPDF add certificate-based digital signatures?

No. SigPDF places your drawn, typed or image signature on the page, along with initials, dates and text. It does not use digital IDs or certificates, and it does not create an audit trail.

How can I tell if a PDF already has a digital signature?

Open it in Adobe Acrobat Reader on a computer and select Signatures in the right panel. Certificate signatures are listed there with their status. A drawn or typed signature that is only part of the page does not show up in that list. If the list shows someone else's certificate signature, do not sign the file in SigPDF: it saves a new copy of the file, and their signature will most likely show as invalid. Ask the sender how they want you to sign.

Can I add a certificate signature to a PDF I signed with SigPDF?

Usually, yes. Download the signed PDF from SigPDF, open it in Acrobat Reader and follow the steps in Part 2. The certificate signature goes on last.

Sign your PDF now

If your signature on the page is what you need, open SigPDF, drop in the file and sign it in your browser. Free to try. Downloading the signed PDF needs a plan: €2.99 a month or €24.99 a year, cancel anytime.

Sources: Adobe Help Center, "Add digital signatures", "Create self-signed digital IDs", "Validate digital signatures", "Prepare for certified digital signing", "Permissions and limitations of signed PDFs" and "About certificate signatures in Adobe Acrobat" (checked 2026-10-24); 15 U.S.C. §§ 7001, 7003 (ESIGN Act); Uniform Electronic Transactions Act, section 7; Regulation (EU) No 910/2014 (eIDAS), Article 25 and recital 49; UK Electronic Communications Act 2000, section 7. General information, not legal advice.

How to Digitally Sign a PDF: With or Without a Certificate