I had a similar thought and "solved" the problem by running a webserver, where you can download the manual to another device by connecting to the retropie.
I write "solved" because it's very hacky and I've only implemented it for DOS and ScummVM games, as games on therse platforms usually need more "exposition" to enjoy properly (or even at all with some of the copy protections).
I haven't worked on this any further as I'm lacking the skills. But at some point I will probably refine the project, if I find the time.