Is typing allowed on Chol Hamoed? Does it make a difference if whatever is typed will be printed afterwards?
Shmiras Shabbos k'hilchasa [II 68:(125)] quotes R' SZ Aurbach that writing on a blackboard is permitted for the following reasons:
It stands to be erased and is very easily erased (true on a computer when the file gets closed or the screen gets turned off or the email gets sent. The fact that it is stored in memory is not an existence of writing).
Since we aren't makpid to write nicely it's only maase hedyot (and so is pushing keyboard buttons).
Others argue with point 1 since the writing will not erase by itself similar to the Rema's wax tablet.
However, if you agree with point 2, you can certainly send an email that is necessary for the moed.
I heard a shiur given by Rav Moshe Heinemann of Baltimore (probably about 15 years ago) in which he mentioned that typing on Chol HaMoed is permitted. The reasoning he gave was that the act of typing is not a Ma'aseh Uman (professional act, forbidden on Chol HaMoed) as just about anyone is capable of typing. Rabbi Heinemann pointed out that although the results of something typed by an expert typist versus that of by a single-finger-typist are the same, this is irrelevant as the skill itself -- i.e., pressing keys on a keyboard -- is not itself a Ma'aseh Uman.
(For example, even someone illiterate can type by matching/copying characters off of another typed or written source. Note: This specific point/example is my own extrapolation that Rabbi Heinemann did not mention (to my recollection) but which I think proves this line of reasoning that typing is not a "skill of expertise", aka Ma'aseh Uman.)
Dose of Halacha bring sources that allow it and those that forbid it based on different reason than mentioned above:
Whether printing from a computer is considered ma'aseh hedyot or ma'aseh uman is the source of much debate. The dilemma is that while certainly nowadays typing requires less skill than writing (see Chol Hamoed Kehilchaso 6:89), nonetheless printed works can look more professional than written ones. Is it the act or results that make the difference?
R’ Moshe Sternbuch (Moadim Uzmanim 4:301) considers printing to be ma'aseh uman and therefore assur.
R’ Moshe Feinstein (Hilchos Chol Hamoed Zichron Shlomo p78) held that typing cannot be considered ma'aseh uman and providing it was of relevance to Yom Tov, would be permissible. R’ Shlomo Zalman Auerbach (Shemiras Shabbos Kehilchasa 66:n209) compares it to using stamps which the Eshel Avraham allows. While storing data to a disc is improving the disc, and therefore tantamount to boneh, providing the work was necessary for Yom Tov (or would cause a significant loss, etc.), it would be muttar. In a letter to R’ Avraham Avraham (Nishmas Avraham OC 4:340:4) R’ Shlomo Zalman explained that displaying letters isn’t problematic, as it merely shows light. R’ Ovadia Yosef (Yabia Omer OC 8:48) paskens leniently, too for the same reason.