2008/3/8, Taco Hoekwater
I doubt it would be possible to do, that even with changes to pdftex. In any case you end up with either no embedded fonts at all, or a rather massive PDF with each page having its own font subsets and images.
pdfTeX would have to generate incremental PDF (3.4.5), and the changes would be non-trivial (fonts, images, page tree, objects, ...), while the benefits are probably of interest for only a small number of users (as the normal user doesn't have such large documents).
But if I could send a signal to my PDFTeX/LuaTeX process, and this would cause it (at least if it would not currently be in a \shipout) to flush the current buffers, create a copy of the current PDF/DVI file under a snapshot name, properly finish/close it (writing reference table/DVI trailers) and then resume regular work, this would be quite dandy.
The idea is interesting, but I have absolutely no time to implement it. I fear that getting the details right may take quite some time (forking is easy, the tricky bit is duplicating the open files and file handles).
Yes, forking would be a way. But I would not implement it in pdfTeX anymore. Definitely luaTeX. Best Martin