Even just basic text is... interesting in PDFs. A few years back, I created a PDF which displayed its own MD5 hash by making every single letter a separate chain of sub-documents and using MD5 collisions to change which sub-document each pointed to without changing the hash. Pretty much every PDF reader managed to interpret this cleanly as ordinary, copy-and-pasteable text because it wasn't any worse than they could expect to encounter in an ordinary PDF, and they all had robust heuristics for dealing with these kinds of shenanigans. (The exception I found was PDF.js, possibly due to the fact it was rendering the whole thing to HTML.) The only real issue was that every PDF reader had a slightly different idea of what characters I could safely use in the names of those nested documents.