There is no mention of AGPL anywhere in that repository. There is just a LICENSE file with a completely different proprietary license: it is not open source at present. If you intend to make it open source, you need to change the license to an open source one.
As Nick says the project is open source (https://github.com/PDFTron/webviewer-ui).
Express is available under under dual AGPL/commercial license ( similar to iText and lot's of other popular OSS )