HexaPDF has quite a lot of documentation and it is organized in the following way to make it easier for you to find what you need:
Tutorials - start here
Introductions to various parts of HexaPDF, like creating a PDF document.
Practial step-by-step instructions for the most common use-cases; they assume familiarity with HexaPDF.
Background information and explanations of some of the key concepts in HexaPDF.
Technical reference material, for API, CLI command and other aspects.
Documentation that doesn’t fit into one of the above categories, like the changelog can be found at the bottom of the side menu.