Happy new year! Sorry for the long delay, the team was all away for the holidays.
PDF.js Express requires that the license key is validated on our server. If the key cannot be validated for any reason, then the document will still work but it will be watermarked even if you have a valid key.
If you need a product that does not “phone home” for license key validation, our sister product PDFTron WebViewer does not phone home - you can check that out if you want. It has the exact same API as Express so you just need to swap dependencies.
We are also going to be releasing an OEM version of PDF.js Express very soon that will not phone home either. If you are interested in that, I can let you know when it’s ready.
Thanks!
Logan