When you write a PDF library, there are design trade-offs all the way down, depending on use cases. (Just “in-memory” is already an important design trade-off, because the PDF format is intentionally designed to not require the whole PDF to be loaded into memory at once.) It would also be antithetical to preferring deep modules with minimal interfaces over shallow modules with broad interfaces [0]. Lastly, in managed environments like the JVM, a C-interface library would come with additional complications and overheads.

[0] https://dev.to/gosukiwi/software-design-deep-modules-2on9