First of all, there is the paper. It must be authentic and from the right time. Watermarks, etc, have trapped many forgers. So have fibers in the paper. The Hitler diaries, for example, were written on paper that was not produced until after WW2.

Next there is the ink. Ink composition has changed rapidly in the past decades. Lead, once common, is now forbidden.

Third, there is the typewriter. It is hard to even FIND working typewriters from the 1980s. Typewriters all have characteristic idiosyncrasies such as letter alignment, flaws in individual letters, etc.

Fourth, the language. It is very easy to use words or expressions that had not yet entered the language at the time the document was supposed to have originated.

Finally, the signature. Signatures are especially difficult to forge, especially when the writing implement is taken into account.

In short, document forgery is for the EXTREMELY expert only. Amateurs need not apply.