Hi,
This is a bit of a vague question but I will do my best.
To save the PDF as a blob, you can follow this guide. You will then want to send that blob to your server and then you can do whatever you wish with it. Note that the annotations WILL NOT BE SAVED into the document, as Express does not support this feature.
To export the annotations as XFDF, you can call:
const xfdfString = await instance.annotManager.exportAnnotations({ links: false, widgets: false });
You can than either store that XFDF in a database, or merge it into the document on your server using some other library.
Alternatively, you can merge the XFDF using our rest api.
Thanks,
Logan