The pdf_compositor service should composite multiple raw pictures from different frames into a complete one, then converts it into a pdf file within an isolated sandboxed process. Currently, it has no compositing functionality, just convert a set of raw pictures into a pdf file within the sandboxed process.

While the compositor creates single-page PDF objects it can optionally collect those into a multi-page PDF document object. Otherwise a multi-page PDF document is made by sending an extra multi-page metafile which contains repeats of each of the previously processed pages all in one larger message.

Message flow when interacting with the PDF compositor is as follows: