Throw It Out
True, the ideas in the document that was unearthed may be helpful but it has been seven years since you've seen it and you functioned well without it all that time and - who knows? - perhaps even better than if you'd had it and now that it's in the trash if a related project arises and you regret having thrown out the paper just remember that some of it stayed with you and you have the ability to start again if every damned paper in your files went up in smoke or were suddenly erased.
Live bravely. Throw it out.