eSign PDFs with a Certificate on Android

class MyActivity :
    PdfActivity(),
    OnSignaturePickedListener,
    DocumentSigningListener {

    private var signatureFormFieldName: String? = null

    override fun onSignaturePicked(signature: Signature) {
        val document = document ?: return
        // Signature form name can be obtained from an `addOnFormElementClickedListener`.
        val signatureFormFieldName = this.signatureFormFieldName ?: return

        val formField = document.formProvider.getFormFieldWithFullyQualifiedName(signatureFormFieldName)

        // The signature object provides convenient conversion to ink or stamp annotation.
        val signatureAnnotation: Annotation = when (signature.annotationType) {
            AnnotationType.INK -> signature.toInkAnnotation(
                document,
                formField.formElement.pageIndex,
                formField.formElement.boundingBox)
            AnnotationType.STAMP -> signature.toStampAnnotation(
                document,
                formField.formElement.pageIndex,
                formField.formElement.boundingBox)
            else -> throw IllegalStateException("Unhandled Signature type. Neither Ink, nor Stamp.")
        }

        // Add the annotation to the document. This step is required so we can render the signature
        // to pass the render to the `SignatureAppearance`.
        document.annotationProvider.addAnnotationToPage(signatureAnnotation)

        val w = kotlin.math.abs(signatureAnnotation.boundingBox.width().toInt())
        val h = kotlin.math.abs(signatureAnnotation.boundingBox.height().toInt())
        val signatureBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888)
        signatureAnnotation.renderToBitmap(signatureBitmap)

        // Now that we've made the bitmap, we can remove the annotation from the document.
        document.annotationProvider.removeAnnotationFromPage(signatureAnnotation)

        // Now sign the document with a certificate asset in the next function.
        signDocumentWithSignatureBitmap(formField.formElement as SignatureFormElement, signatureBitmap)
    }

    private fun signDocumentWithSignatureBitmap(formElement: SignatureFormElement, signatureBitmap: Bitmap) {

         val path = FileUtils.getTempFilePath(applicationContext, "pdf")
         val temporaryFile = File(path ?: "")

         val signatureAppearance = SignatureAppearance.Builder()
               .setSignatureGraphic(SignatureAppearance.SignatureGraphic.fromBitmap(getImageUri(signatureBitmap)))
               .build()

         val key = getPrivateKeyEntry(applicationContext)
         val signedDocumentUri = Uri.fromFile(temporaryFile)
         val signerOptions = SignerOptions.Builder(formElement.formField, signedDocumentUri)
               .setSignatureMetadata(DigitalSignatureMetadata(signatureAppearance = signatureAppearance))
               .setPrivateKey(key)
               .build()

         // Handles the signing process.
         SigningManager.signDocument(
               context = applicationContext,
               signerOptions = signerOptions,
               type = DigitalSignatureType.CADES,
               onFailure = {
                        // Handle signing errors here.
                       }
                    ) {
                       // Replace the loaded document with the signed document.
                       setDocumentFromUri(signedDocumentUri, null)
                }
    }
}