These articles provide background information and explanations of some of the key concepts of PDF and HexaPDF.
PDF Object Representation
Information about the types of PDF objects and how they are represented in HexaPDF
How a PDF document is structured and the most important objects
How encrypting PDFs works in general and how to use it with HexaPDF
Information on interactive PDF forms and how to use them with HexaPDF
Information about digital signatures with respect to PDF