Thank you for using Syncfusion products.
IE rendering engine make use of MSHTML (IE rendering engine) to convert HTML to vector images. From the images, we will render the PDF document. The alternate text information is not added in meta file, so we could not extract the alternative text details of the image inside the meta file. IE rendering engine does not supports adding alternate text for images inside the HTML string. Also, currently we do not have support for adding alternate in the converted/existing PDF documents.
Please let us know if you need any further assistance on this.