Solutions · Document Intelligence · Digital signatures

Sign PDFs locally. Verify them honestly.

Digital signatures usually mean a signing service and documents leaving your infrastructure. LM-Kit ships the whole chain in process: PdfSigner produces PAdES and PKCS#7 signatures with certification, RFC 3161 timestamps, and long-term validation; PdfSignatureValidator verifies any signed PDF against your own trust anchors. Keys load from files, OS stores, or hardware, and can stay in an HSM entirely.

Start building free PdfSigner API

PAdES + PKCS#7 RFC 3161 timestamps LTV / DSS 100% local

`PdfSigner`

One-call signing, sync and async. The certificate's private key may live in a PFX file, the OS store, or hardware-backed storage.

`PdfSignatureValidator`

One verdict per signature: Valid, Indeterminate, or Invalid. Integrity, identity, revocation, and timestamp judged separately.

External signing

BeginSign prepares the document; your HSM or signing service returns the CMS container. Private keys never touch the process.

The full signing lifecycle.

Sign, certify, countersign, timestamp, and keep signatures verifiable for decades. Every operation runs on your machines.

Standards

PAdES and PKCS#7

PAdES detached signatures (ETSI.CAdES.detached) by default, classic PKCS#7 when a legacy workflow requires it. Standard PDFs, readable everywhere.

Revisions

Incremental by design

Signing appends an incremental revision, so the original bytes survive verbatim and existing signatures keep verifying. Countersigning is just signing again.

Certify

Certification signatures

Certify a document with DocMDP permissions: forbid all changes, or allow form filling and annotations while the certification stays intact.

Time

RFC 3161 timestamps

Attach a timestamp token from any TSA to a signature, or add standalone document timestamps that prove the file existed at a point in time.

LTV

Long-term validation

ExtendLtv embeds certificates and revocation data into the document security store, so signatures still verify after certificates expire.

Visible

Signature appearances

Visible marks with text and images, auto-composed in 14 languages, placed by exact bounds. Embed your own font for non-Latin scripts.

Verify

Trust you configure

Verification chains each signer to trust anchors you control and reports whether the signature covers the whole document or an earlier revision.

Agents

Agent-ready tools

Built-in tools (pdf_sign, pdf_signature_verify, pdf_signature_list, pdf_document_timestamp, pdf_ltv_extend) put signing under agent permission policies.

Three working pipelines.

One call signs the document with a PAdES signature. The original bytes survive verbatim in the signed file.

SignContract.cs

using LMKit.Document.Pdf;

// The certificate's private key may live in a PFX file,
// the OS certificate store, or hardware-backed storage.
var options = new PdfSigningOptions
{
    Certificate = certificate,
    Reason      = "Approved",
};

PdfSigner.Sign("contract.pdf", "contract-signed.pdf", options);

Verification returns one result per signature, with integrity, identity, revocation, and coverage judged separately.

VerifySignatures.cs

using LMKit.Document.Pdf;

var report = PdfSignatureValidator.Validate("contract-signed.pdf");

Console.WriteLine($"Overall: {report.OverallStatus}");

foreach (var result in report.Signatures)
{
    Console.WriteLine($"#{result.Signature.Index}: {result.Status}");
    Console.WriteLine($"  Integrity: {result.Integrity}, identity: {result.Identity}");
    Console.WriteLine($"  Covers whole file: {result.Signature.CoversEntireDocument}");
}

Timestamp the signed file through your TSA, then embed revocation material so it verifies long after certificates expire.

TimestampAndLtv.cs

using LMKit.Document.Pdf;

// RFC 3161 document timestamp from your TSA.
byte[] stamped = PdfSigner.AddDocumentTimestamp(
    File.ReadAllBytes("contract-signed.pdf"),
    new PdfTimestampOptions { TimestampAuthority = new Uri("https://tsa.example.com") });

// Embed certificates and revocation data (DSS) so the
// signature stays verifiable after certificates expire.
byte[] archival = PdfSigner.ExtendLtv(stamped);

File.WriteAllBytes("contract-ltv.pdf", archival);

Three verdicts. No wishful thinking.

Verification follows the three-state model professional viewers use, so a verdict you store is a verdict you can defend.

Valid

Proven good

The document bytes match the signature and the signer chains to a trust anchor you configured.

Indeterminate

Unproven, never assumed

Nothing proves the signature broken, but it could not be fully verified: an untrusted signer or missing information. Reported as exactly that.

Invalid

Proven broken

The document was modified after signing, the signature value does not verify, or the signer certificate is revoked.

Embed it, or serve it.

The same signing engine ships in LM-Kit One as REST endpoints: sign, verify, timestamp, and LTV-extend PDFs from any language over HTTP.

LM-Kit One, the Private AI Application Server

Signatures plus the toolkit.

PDF/A conversion and validation

Archive what you sign: PdfAConverter produces ISO 19005 output and PdfAValidator checks any file's claim independently.

PDF/A page

PDF redaction

PdfRedactor permanently removes sensitive content before the document is signed and released. Content is deleted, not covered.

PDF redaction page

PDF toolkit

Merge, split, render, search-highlight, unlock, and inspect PDFs with the same library.

PDF toolkit page

Build it. Read it. Try it.

Working console demos on GitHub, step-by-step how-to guides on the docs site, and the API reference for the classes used on this page.

Demo

Signed on your machines. No signing service.

Free Download PdfSigner API