Can we extract, save, merge annotations data in our database without sending pdf data to Pdfjs Express server
You can extract and save without using the REST API, but it will only work for new annotations. If the user opens a PDF that has annotations already in it, you will not be able to extract those without the REST API.
Merging is not supported at all without the REST API.
If you need this functionality without using our servers, take a look at our sister product, PDFTron WebViewer. It offers all this functionality client side.
Let me know if you have any more questions.