How to Programmatically Stamp and Digitally Sign a Document on Android

// Our PDF activity we'll use to stamp and sign documents.
// See our website guides for how to set up a `PdfActivity` and load a document.
class MyPdfSigningActivity : PdfActivity() {

    ...

    // Creates a signature form field of size `0` that can be used to digitally sign a document
    // without adding any signature appearance.
    private fun createInvisibleSignatureField(
            pageXPosition: Float,
            pageYPosition: Float,
            pageIndex: Int,
            formFieldId: String) {
        // Bounding box of size `0` at the specified page coordinates.
        val bbox = RectF(pageXPosition, pageYPosition, pageXPosition, pageYPosition)
        val signatureField = document?.formProvider?.addFormElementToPage(
            formFieldId,
            SignatureFormConfiguration.Builder(pageIndex, bbox).build()
        )
    }

    // Our main calling code can pass in the page coordinates and page where we want our stamp to be:
    private fun stampAndSign() {
        // We want the signature around the bottom-right corner of the page. We use PDF points as the unit for coordinates.
        // Remember the
        `createInvisibleSignatureField(`.
            pageXPosition = 500f,
            pageYPosition = 200f,
            pageIndex = 0,
            formFieldId = "SignatureStampField"
        )
    }
}

// Place the stamp image at the location of the invisible form field created previously.
// Pass the form field ID as an argument so we can reuse this function to add the stamp
// to multiple signature locations in the document.
fun addStampToDocumentPage(formFieldId: String) {
    // Load the image from assets into a bitmap.
    val image = BitmapFactory.decodeStream(assets.open("images/stampImage.png"))

    // Our asset is too big (~900×600 pixels),
    // so we can shrink it 6× to better fit the page.
    val shrinkCoefficient = 6
    val imageWidth = image.width / shrinkCoefficient
    val imageHeight = image.height / shrinkCoefficient

    // Find the form field we made earlier.
    val signatureField = document?.formProvider?.getFormElementWithName(formFieldId)

    signatureField?.let {
        val formBbox = it.annotation.boundingBox

        // Create the annotation at the location of the form field.
        val stamp = StampAnnotation(
            it.annotation.pageIndex,
            RectF(
                formBbox.left,
                formBbox.top,
                formBbox.left + imageWidth,
                formBbox.top - imageHeight),
            image
        )

        // Mark it as a signature.
        stamp.setIsSignature(true)

        // Add it to the document.
        document?.annotationProvider?.addAnnotationToPage(stamp)
    }
}

// Load the certificate from the assets folder and add it to the signature manager.
// We return the `Signer` for use later when signing the document.
fun loadCertificate(): Signer {
    // The signer is a named entity holding a certificate (usually a person) and has a display
    // name shown in the app.
    val signer = Pkcs12Signer(
        "John Appleseed",
        Uri.parse("file:///android_asset/JohnAppleseed.p12")
    )

    // We need to register the signer with the signature manager for signing to work.
    SignatureManager.addSigner("john_appleseed", signer)

    // Provide a password to the signer, which will be used to unlock its private key.
    if (signer is InteractiveSigner) {
        (signer as InteractiveSigner).unlockPrivateKeyWithPassword("test")
    }

    return signer
}

// Sign the document using a certificate located in the assets directory.
fun digitallySignDocument(formFieldId: String) {
    val signer = loadCertificate()

    // Find the form field we made earlier.
    val signatureField = document?.formProvider?.getFormElementWithName(formFieldId)

    // Show the signer dialog that handles the signing process.
    SignatureSignerDialog.show(
        supportFragmentManager,
        SignatureSignerDialog.Options.Builder(document, signatureField, signer).build(),
        this
    )
}

class MyPdfSigningActivity :
    PdfActivity(),
    DocumentSigningListener {

    // Implementation of the abstract listener function.
    // Reloads the digitally signed document.
    override fun onDocumentSigned(signedDocumentUri: Uri) {
        setDocumentFromUri(signedDocumentUri, null)
    }

    ...

class MyPdfSigningActivity :
    PdfActivity(),
    DocumentSigningListener {

    fun stampAndSignPdf() {
        // An ID for the invisible form field we create to place our stamp.
        val formFieldId = "SignatureStampField"

        // Create the signature field.
        createInvisibleSignatureField(
            pageXPosition = 500f,
            pageXPosition = 200f,
            pageIndex = 0,
            formFieldId = formFieldId
        )

        // Add the signature stamp image.
        addStampToDocumentPage(formFieldId)

        // Load the certificate from our assets.
        val signer = loadCertificate()

        // Sign the document using the new `Signer`.
        digitallySignDocument(signer)
    }

    // Reload the digitally signed document.
    override fun onDocumentSigned(signedDocumentUri: Uri) {
        // Replace the loaded document with the signed document.
        setDocumentFromUri(signedDocumentUri, null)
    }

    ...