I echo DarkLink's sentiment that the risk of digitally altered codices is one that you can really only guard against by having your own reference copies (which you've indicated is not financially feasible - something I quite understand). At the same time, I think that the risk of digitally altered codices (whether digitally altered digital e-books or digitally altered printouts) is small enough that it doesn't really bear losing much sleep over. It's certainly possible to digitally alter a printout so that it looks like you've just copied the relevant pages of your physical codex, and I'm sure it's possible to digitally alter an e-book, but for most people that would not be a trivial task.
In addition to being a small enough risk that I don't think you need to spend too much worry guarding against it, I also think it's a small enough risk that it wouldn't be fair to penalize players for bringing the product. After all, for every person who brings an e-book version of their codex or supplement specifically because they are savvy enough to have altered the file, there are going to be a hundred people (or a thousand) who bring the e-book version because it's lighter and/or they find it more user-friendly.