The PDF Viewer library allows you to enable the resizer for the Text Markup annotations programmatically using the enableTextMarkupResizer property. Default value of the property is false.
Step 1: Follow the steps provided in the link to create simple PDF Viewer sample.
Step 2: Add the following code snippet to enable the resizer for the Text Markup annotation.
// Enable TextMarkup Resizer.
viewer.enableTextMarkupResizer = true;
Find the Sample how to enable the resizer for the Text Markup annotations