Mark Lomas canters through the legalities of protecting an idea whilst pitching to raise money and offers some good suggestions. To my mind, the best of these is the simplest - just mark your documents as "Strictly Confidential". It gives someone thinking to forward the information cause to pause. The sender will know the documents are confidential and that they can be tracked. What's more, the recipient will realise the sender has been naughty.
At a human level, that'll influence behaviour enormously.
Rest assured in the end the investor will want to be sure that you protect your IP, so in the first instance marking all written material strictly confidential and getting a simple agreement verbal or written that the material is sent to them and will be treated as confidential to you is a good basic start.