@Borodin - obviously theducks just missed that part of your post. You have to appreciate that there are an enormous number of repeated posts every week from people who *don't* bother to read the pdf sticky or think that somehow their case is different. You happened to get hit by theducks on autopilot. Birdstrike
If you are wanting to continue the technical discussion along the lines of you adding the feature yourself this thread would be better placed in the Development forum.
Whether that new PDF engine is going to give you much joy in this regard though is another matter. Kovid recently upgraded a number of the pdf components and the latest version that ships with calibre has some terminal issues I found with my Extract ISBN plugin (causing unmanaged code crashes). I changed my plugin to use some different pdf components as did Kovid for calibre as he has other priorities to work on. I am sure he can give you a more definitive answer.