Document signing API · 25¢ per envelope · No monthly fee

Document signing API: tag a PDF, POST it, get a webhook

Get your API key. 25 documents free, no card. Then it's 25¢ per document, flat: tag the PDF, POST it, and the signing link goes out by email and text.

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# Tag the PDF with ^S1 where they sign, then:
B64=$(base64 < agreement.pdf | tr -d '\n')

curl -X POST https://api.esigndev.com/v1/envelopes \
  -H "Authorization: Bearer sig_..." \
  -H "Idempotency-Key: order-41882-contract-1" \
  -H "Content-Type: application/json" \
  -d '{
    "document_name": "Roof replacement agreement",
    "document_base64": "'"$B64"'",
    "delivery": "both",
    "sender": { "company_name": "Your Company" },
    "signer": {
      "name":  "Jane Smith",
      "email": "jane@example.com",
      "phone": "+18135551212"
    }
  }'

# POST to your webhook endpoint when the last signature is in
{
  "id": "evt_8f3k2m9q1x",
  "type": "envelope.completed",
  "occurred_at": "2026-09-28T15:04:11.482Z",
  "data": {
    "envelope": {
      "id": "env_k7m2v9x3q1",
      "state": "completed",
      "document_name": "Roof replacement agreement",
      "delivery": "both",
      "signer": {
        "name": "Jane Smith",
        "signed_at": "2026-09-28T15:04:09Z"
      },
      "completed_at": "2026-09-28T15:04:09Z"
    }
  }
}

# then fetch the signed PDF, certificate included:
curl https://api.esigndev.com/v1/envelopes/env_k7m2v9x3q1/document \
  -H "Authorization: Bearer sig_..." -o signed.pdf

One POST creates the envelope and sends it. Bearer token, no JWT, no consent screen. The webhook body is trimmed to the fields you act on; the full shape is in the docs.

25¢

per envelope, flat at every volume. No monthly fee, no seats

SMS

and email, or both, in one API call, included in the 25 cents

ESIGN

and UETA, with a certificate page for each signer on every signed PDF

30 days

signed copies are kept after the envelope closes, then deleted

Tag, then post

The PDF you already generate is the input

Type ^S1 where signer one signs and ^D1 where their date goes. Set that text in white on a white background and it disappears for the reader while staying in the PDF text layer where we can find it. Inline base64 covers files up to about 3 MB, which is most generated contracts in a single call. Larger files, up to 15 MB, go through a two-step upload.

How it works

What the document signing API does with your file

We read the text layer, place the fields where your tags are, and send. Your layout stays yours, and changing the document does not mean re-dragging boxes.

Tag the PDF

Put ^S1 where the signature goes and ^D1 where the date goes, in white text. Invisible to the reader, findable by us.

POST it

One JSON request with the file and the signer. Delivery by email, text or both starts immediately.

Get the webhook

envelope.completed lands on your endpoint with the envelope id. Download the signed PDF, certificate included, and keep it.

After the POST

What happens after the POST

The envelope is created. The response carries its id, beginning env_, and the signing link.
The signing link goes out by email and text, from the same call, at the same 25¢.
The signer signs on their phone. They tap the link in the text, read the document and sign.
A webhook fires on each event: envelope.delivered, envelope.viewed, envelope.completed, and envelope.partially_signed when the first of two signers finishes.
On envelope.completed you download the signed PDF, with its certificate page, and keep your own copy.

Enterprise

Collect typed answers before signing. Get them back as JSON.

Ask for the policy number, the date of birth, the spouse's name. The signer answers on their phone, signs once, and every answer is printed on the document and sent to your system.

Typed answers, dropdowns, checkboxes and choices, required or optional, placed in the PDF where they belong. Phone, email and date formats are checked as they type.
Rules: show or require a question based on an earlier answer, or require exactly one, at least N or at most N across a set. The server checks them again at signing, so they can't be skipped.
Printed on the signed PDF, each answer exactly where it goes.
A SHA-256 fingerprint of the answers on the signing certificate, not the answers themselves.
Returned as JSON on the envelope and in the envelope.partially_signed and envelope.completed webhooks.

Data collection is on Enterprise accounts. See how it works or talk to us. Everything else on this page is 25¢ an envelope.

Pricing

What it costs

What	Siglio
Per envelope	25¢
Monthly fee	$0
Per seat	$0
Minimum	$0
SMS delivery	Included, same 25¢
Free trial	25 documents free, no card

You are billed when an envelope is created, not when it is signed. A declined or ignored envelope still costs 25¢.

Next to the market

Service	Per envelope	SMS delivery	Monthly fee
Siglio	25¢	Yes, included	None
eSignatures.com	49¢	Yes, 9¢ extra per text	None
Documenso	Not published	Not mentioned	$25 to $250/mo, or self-host
DocuSign	Not published	Not on the Starter or Intermediate API plans; eSignature add-on from $0.36 per delivery	$50 to $480/mo (API plans)
Adobe Acrobat Sign	Not published	Not published	Not published for API tiers
PandaDoc API	Not published	Email or SMS listed, price not published	$40/mo Developer plan, 40 documents

Each company’s figures are from its own published pricing page, linked from its name, checked 7 October 2026. DocuSign’s SMS add-on is from its eSignature plans page; its API plans list SMS as “Does Not Include” on Starter and Intermediate. Adobe’s Acrobat Sign API FAQ says API access is “reserved exclusively for enterprise and developer tier accounts” and prices neither. Documenso’s page does not mention SMS. “Not published” means the page does not state it. Prices change; check the source before relying on it. Longer comparisons: DocuSign, PandaDoc, Adobe Sign.

Legal standing

Legally binding, with the record to show it

ESIGN ActFederal, 15 U.S.C. §7001 UETA49 states, D.C., Puerto Rico, U.S.V.I.

Electronic signatures have carried the same legal force as ink in the United States since 2000, under the federal ESIGN Act and, at state level, the Uniform Electronic Transactions Act. Every signed PDF carries a certificate page per signer with the sent, viewed and signed times and the IP address each came from. How the record holds up.

Questions

What to know before you send the first PDF

What does 25¢ include?

One envelope: the document, the signing link by email, text or both, and a webhook for every event on it. 25¢ flat at every volume, with no tiers. $0 monthly fee, $0 per seat, $0 minimum.

Is SMS extra?

No. Email and text go out in the same API call at the same 25¢. There is no SMS surcharge.

What do the free documents deliver to?

Real inboxes and real phones. The first 25 documents are free with no card, so the first thing you test is exactly what your signers will get. Your API key is ready in about a minute, with no account review.

How are signed documents returned?

envelope.completed fires on your endpoint, and your handler downloads the signed PDF from the API. Siglio deletes signed documents 30 days after completion, so store your own copy when the webhook arrives. Webhooks are on every account, for every event type.

Is it legally binding?

Yes. Electronic signatures carry the same legal force as ink in the United States under the federal ESIGN Act and state UETA. Every signed PDF includes a certificate page per signer with the timestamps and IP addresses. More on the law.

What are the limits?

PDF only, up to 15 MB, no password-protected files. Up to two signers per envelope; signer two is notified once signer one has finished. Bearer token auth, no JWT and no consent screen.

Send a tagged PDF in the next ten minutes

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or use your work email

25 documents free. No card needed.

Check your email

We sent a sign-in link to . It is good for 15 minutes, and your API key is waiting on the other side.