Eefs,
I don't think you should admit to having read this letter. You were asked to look for something on your colleague's desk. You should not have read the details of a letter that was addressed to him, I would guess you could see almost immediately that it was not the document you were seeking. I would be quite angry if I was him, that you had read it (but he's a chump for leaving it in his desk anyway). I also don't think telling your employers you have read the letter will do your professional standing any good.
The different pay stuff happens all the time. If you try to confront your employers over it they will probably say he had more experience than you/a better degree/wider technical knowledge or any such excuse. The simple fact was that they wanted him for the job, and offered what they thought would make him accept.
As aussiesim says, check your salary with what is common in your industry. If you are being underpaid, then go to the boss with that. If you're getting the going rate, and your colleague is being 'overpaid' then your company will probably expect more of him in the long run anyway (no such thing as a free lunch, you know). He may be on a probationary period with mega targets or something.
Finally ask yourself this. Were you badly dissatisfied with your salary before you read this letter? If the answer is no, then learn the lesson, you don't do yourself any good by snooping around.
Oh, and if none of this helps, sell your services and experience to someone else. You'll probably get paid more than their long-standing employees when you start a new job!