Is XPS Annotator will always be free?

Yes, XPS Annotator will always be free. Although free for you to use, but there are certain limitations that you should do, like no modifications, you cannot sell, rent, lease, or lend the XPS Annotator to another parties.

For more details you can read a license from the XPS Annotator. The license will explained the limits of what you can do and that you should not do.