I don't see any problem in using Reiki on documents of any kind. If you use symbols, you could use Cho Ku Rei as well, either by physically drawing it on to the papers, drawing it with your finger or by projecting the symbol. This may help prevent misfortune and shield from negative energies as well channeling Reiki.
Hematite would be a useful crystal for help in legal matters and is very grounding and protective. Black tourmaline would draw out negativity. Moonstone is helpful for stress and anxiety relief and can help clear confusion. Depending on the exact situation there may be suitable crystals depending on that. For example, if you need the courage to speak the truth or support with communication, crystals associated with the throat chakra would help.