You can update the form field's at runtime using the FormFieldClick event and UpdateFormFieldsAsync() method of SfPdfViewer. The following code example explains how to open Context menu when you click on the form field and how to update the menu item content as form field's value. In this example, the Syncfusion’s ContextMenu component is used to update form field.
Documentation: https://blazor.syncfusion.com/documentation/pdfviewer-2/how-to/update-form-field-using-context-menu
To run this application, you need Visual Studio 2019 or later versions.
To debug while running the application, press F5 or select Debug > Start Debugging. To run the sample without debugging, press Ctrl+F5 or selectDebug > Start Without Debugging.