License
HexaPDF is licensed under the GNU Affero General Public License (AGPL), an open source license. It allows you to use HexaPDF for free as long as you adhere to the license. Additionally, there is also a commercial license.
The AGPL puts some restrictions in place to make sure that the community benefits from changes to the code. For example, if you use HexaPDF in an application and distribute that application, you have to make the source code of the whole application available under the AGPL. The same applies if your application is used over a network. Please refer to the license for details.
If you don't want to adhere to the AGPL or can't (e.g. because you can't make the code that uses HexaPDF available under the AGPL), then you need to purchase a commercial license.