You don't have anything to lose by trying the compressed air. If it doesn't work, then you always have in on hand in case you want to read the contents of a letter in an envelope without opening it (steaming is waaay to obvious). Here's what you do. First, go outside with the envelope and the compressed air. Then turn the compressed air can upside down and spray it at the envelope. This will cause the can to spray this highly toxic liquid that will soak into the paper long enough for you to read through it, but will evaporate quickly. Once you have the envelope fairly saturated, take a picture of it with a digital camera. If the letter is folded you may need to reassemble it's contents in photoshop. Finally, let the envelope dry, and make sure that you never touch the liquid the compressed air can spit out. Like I said, it's really toxic.