You are welcome to contribute to HexaPDF. Please note that HexaPDF is licensed under the GNU Affero General Public License version 3 and only code licensed as such can be included.
Before you contribute please sign the individual contributor license agreement or entity contributor license agreement (for companies and other organizations) and email the signed document to email@example.com. This agreement serves a dual-purpose:
It provides the basis for a possible relicensing later on, without the trouble of tracking down each individual contributor.
It allows me to keep my options open in regard to commercializing HexaPDF at a later date.