1. There is probably only one machine that existed in the early seventies that could have created these memos: The IBM Selectric Composer. But the Composer is not really a typewriter, it is more like a typesetting machine. In 1973 dollars, it would have cost upwards of $16,000.
2. Gerry Kaplan, who operates the only website dedicated to the IBM Selectric Composer, attempted to duplicate a sample from one of the memos on his Composer. Here is the result: 
As you can see, the Composer was able to create the superscript and it is proportionally spaced. So at first glance it looks pretty close to the memos. But here is what you get when you superimpose what the Composer produced on top of the actual memos (red is from the Composer, black is the actual memo): 
As anyone can see right away, these do not match up. The second and fourth lines are completely off. It is becoming increasingly unlikely that the IBM Composer, the most advanced "typewriter" of the time, created these memos. Just to get it as close as he did (and that's still not very close), Mr. Kaplan had to do a lot of manipulating. So much so that it is highly unlikely that a non-typist like Killian would have gone to all the trouble.
3. Another thing that points toward a forgery is the perfect centering of the head. This is what convinced Gerry Kaplan that the whole thing is a hoax. The letterheads on two of the memos (supposedly created months apart) were perfectly centered and when superimposed on one another, match exactly. You couldn't do this on a Composer. It's hard enough to get it so perfect once, but for Killian to be able to do it AGAIN, months later? The final test was to see if this perfect letterhead could be created on MS Word. Here it is, with Word superimposed over the actual memo: 
They match perfectly. This is a scam.