How to get the location of an ink signature for digital signing

If you want to digitally sign a document that doesn’t yet contain a SignatureFormField, you can add one at the location of an ink signature. To get the location of an ink signature, you have three options.

Finding an already added ink signature 

@Override
public void onDocumentLoaded(@NonNull PdfDocument document) {
    // This will return the first ink signature found in the document.
    document.getAnnotationProvider().getAllAnnotationsOfType(EnumSet.of(AnnotationType.INK))
        .cast(InkAnnotation.class)
        .filter(InkAnnotation::isSignature)
        .firstElement()
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(inkAnnotation -> {
            digitallySignAtLocation(inkAnnotation.getBoundingBox());
        });
}

Finding an ink signature that was just added 

@Override
public void onDocumentLoaded(@NonNull PdfDocument document) {
    super.onDocumentLoaded(document);
    document.getAnnotationProvider().addOnAnnotationUpdatedListener(new AnnotationProvider.OnAnnotationUpdatedListener() {
        @Override
        public void onAnnotationCreated(@NonNull Annotation annotation) {
            if (annotation instanceof InkAnnotation &&
                ((InkAnnotation) annotation).isSignature()) {
                // This will be called when the user adds a new ink signature.
                digitallySignAtLocation(annotation.getBoundingBox());
            }
        }


        @Override
        public void onAnnotationUpdated(@NonNull Annotation annotation) {


        }


        @Override
        public void onAnnotationRemoved(@NonNull Annotation annotation) {


        }
    });
}

Finding an ink signature that the user selected 

@Override
public void onDocumentLoaded(@NonNull PdfDocument document) {
    super.onDocumentLoaded(document);
    getPdfFragment().addOnAnnotationSelectedListener(new AnnotationManager.OnAnnotationSelectedListener() {
        @Override
        public boolean onPrepareAnnotationSelection(@NonNull AnnotationSelectionController controller,
                                                    @NonNull Annotation annotation,
                                                    boolean annotationCreated) {
            // You can return `false` to prevent the default annotation selection from happening.
            return true;
        }


        @Override
        public void onAnnotationSelected(@NonNull Annotation annotation, boolean annotationCreated) {
            if (annotation instanceof InkAnnotation &&
                ((InkAnnotation) annotation).isSignature()) {
                // This will be called when the user selects an ink signature.
                digitallySignAtLocation(annotation.getBoundingBox());
            }
        }
    });
}

See here for information on how to create digital signatures.