To customize the annotation selector in Syncfusion PDF Viewer, you can use the annotationSelectorSettings property of the PdfViewer control.
Here is an example of how you can customize the selector of the shape annotation:
<button id="annotationSelector">annotationSelector</button>document.getElementById('annotationSelector').addEventListener('click', () => {
viewer.rectangleSettings.annotationSelectorSettings.resizerShape = 'Circle';
viewer.annotationModule.editAnnotation(viewer.annotationCollection[0]);
});Find the sample how to customize the annotation selector