It depends.<p>Let's say you are negotiating about an offer for - say - 100,000 units (make them dollars, euros, peanuts, whatever).<p>How big is the competing offer?<p>110,000, 130,000 or 250,000 units?<p>You are - presumably - to be hired by someone that is ok about giving you a certain (huge) number of units in exchange for your work and - implicitly - is trusting you.<p>So, if you actually have a 110,000 offer letter, you should say that (and show no letter).<p>If you have a 130,000 offer letter, you should say that (and show no letter) but you will likely be asked to show the letter and then it depends on you whether you think that they want to see it because they will make a counter offer or they want to see it only to know who is the one that is going to spend 30% + (both imply that they don't trust you much).<p>If you have a 250,000 offer letter, you are not negotiating, you are losing a better job.
Don't share letters. There is no reason to.<p>Whether they might think you're bluffing or not, at the end of the day they are prepared to go up to a point but are simply trying to get you for as cheap as possible. There is no qualm to be had in getting them to pay the max. they are prepared to.<p>Their attitude on this is also something for you to consider because it tells you something about how working for them might turn out to be.
Consider redacting anything other than the salary that may be sensitive, but yes, you can share them.<p>Depending on how you negotiate, you may benefit from not sharing numbers and letting the other side guess (potentially overshooting and offering a much higher salary).