Hi Chitrang,
Thanks for contacting us regarding PDFJS Express!
Fortunately, the information that quads contain is good enough to programmatically create a highlight annotation.
We have a text-position sample that shows the code you might be interested in. The sample is available under the samples/advanced/text-position folder in the download package from https://pdfjs.express/documentation/get-started/manually.
The function in the sample that you want to take a look at is highlightText.
Let me know how this works and if you have any other questions!
Best Regards,
Zhijie