It is likely that you encountered this term as a watermark or a source for other books. For instance, it has been linked to:
: Authors and publishers maintain absolute control over typography, margins, and embedded diagrams, ensuring the reader sees exactly what was intended. purepdfbook
Key features of the core purePDF library include advanced text rendering, graphics support, gradients, transparency, annotations, and interactive forms. The library comes packaged as an SWC class library, and developers typically use it by adding the library to their project and then writing ActionScript code to generate PDF output. It is likely that you encountered this term