Hi everyone,
PDF.js Express does support range requests by default (only for PDFs) and will use them whenever available. There is no additional configuration required on the client as it is enabled by default. However, the PDF file must be linearized and your file server must be set up to support range requests as well.
That being said, any functionality beyond what is being provided will be tricky as this is mostly handled by the internals of the core PDF.js library. If you absolutely require these more advanced use cases (streaming encrypted files etc), then PDFTron is your best bet.