Najwiekszym problemem jest szybkość a nie utf czy inne dziwadła. Dzisiaj ludzie już nie pamiętają, ale kiedys komputery reagowały dużo szybciej. Klawiatura nic tu nie zmieni, trzeba zmienić system

Back in the early 90s when I first learned to program at university, this was a standard assignment: to program a text line editor. There was a commercial program called "edline".
Anyway, I would bet GitHub would have something if  you prefer to modify someone else's code rather than write your own. The downside is wading through the all the accounts to find one that is short and neat.
From a space point of view I don't think the ATMEGA328 is too small.
The first one I found was upman/Snap-Edit but I am sure there would be better if you keep looking.
Regards AlanX

Actually arduino with 32kb flash is enough to store such project. But it's not "a little more complicated than blinky led". First part (line editing and printing) would take me about 2-4 days of full-time work. Second part would take me 2-4 weeks (RTF and big document support). Would you like to work for free for several days or weeks?
 Also your project is not interesting.

Ignoring the last sentence of your comment because it's quite rude... I have actually considered posting a bounty for this project if I can somehow scrape together the money to do so. However, given the responses I've gotten so far, I don't think I'll be doing so here...

So wait let me get this straight, you want someone to do all the hard work for you (of which you take no interest in learning yourself), stipulate all the conditions to your benefit and then maybe just maybe if someone is crazy enough to put in the non-inconsiderable man hours making something solely for the benefit of a stranger you "may" send something nice?

Think if your work expected the same from you. Your boss: "Yeah I'm gonna need you to start working 16 hours days (including weekends), and after a few months we might consider paying you for the work ...". Why is it that, technical skilled sw/hw folks are just about the only group expected to work for free (or worse at their expense)? I am put in this position all the time and my response is always the same: if my time, experience and value is so low in your estimation then it should be a cakewalk for you to just learn and do it yourself.

I'm not trying to discourage or be disparaging to you, but this mentality is going about it in completely the wrong way and wont get you very far in your goal. It demeans and devalues those who are highly skilled and hardworking (who are in fact doing you a favor, not the other way around) and frankly is insulting to those of us who spent years and years developing non-trivial skills.

Instead, a more effective way would be to partner with someone where you split up the responsibilities (you learn some parts that you can do to split the load) and work out clearly at the start a deal where both parties will get some benefit. If you want to have it all one sided where you don't do any coding and the other guy does all the work, then the only way that will happen is if you commission the work and pay the engineer his hourly wage or fixed payments upon completion of key parts of the project.

Update in reply to OPs snarky reply to valid criticism: Asking for help while (I am assuming unintentionally) insulting/demeaning those you are asking for said help and then acting surprised when they don't welcome you with open arms is being rude in my opinion. Also asking for someone to essentially "do all the hard work" is not asking for a little help, it is asking for the whole enchilada. And then turning around and further insulting those who bothered to reply is low. I explained as nicely as possible how your request was problematic and even made some suggestions on how to approach it to get better results. Playing the victim wont help, but learning how to effectively approach others when you need help (and understand their position as well) will.

Or just buy one of those electric typewriters that do exactly what you describe, can be bought cheaply used, and even already come in a case.

The problem with the portable word processors I've seen (a standard electric typewriter does not have the display) is that they don't have a full-line (80 character) display...

This is not going to work for many reasons:

The Arduino is too small to fit all the code.

This is mainly a software project and if you are not willing to do any coding it is not your project and not going to happen (most likely).

The obvious solution to what ever problem you are trying to solve is to get a small computer like a Raspi that can run a proper OS that already comes with all the software you need for this. Hook up a proper display. If you don't want a full size monitor use a small LCD or OLED display.

I don't want a RasPi. A RasPi in this applicaiton is using a windmill to swat a fly.

Look up the Kaypro II computer some time. It had astonishingly limited specs compared to eg a RasPi. Arthur C Clarke used one to write entire novels. If a Kaypro II can do it, an Arduino ought to be able to...

You, on the other hand, want someone to swat a fly with chopsticks for you. Just check how much RAM an ATmega328p has, and then calculate how many characters that translates to. Of course you would need Unicode support and formatting and so on, and you will need several buffers, so in practice it will be about twice as much or more, but even that simple calculation will give you an idea.

I think that you are also greatly underestimating the difficulty of programming a text editor. There are literally whole books written on the subject -- maybe reading one would give you a better idea.

Unicode? Why not standard ASCII? And a simple navigable 160byte text buffer (a two-dimensional array with each character as an element, paired with a second character that represented its formatting attribute) would do the job of holding everything. It would be clunky and kludgy, but it *would* work. As for disk access and the like -- it would pretty well have to write each line to disk and to the printer more-or-less at once, and retrieve lines one-at-a-time from disk to scroll through a document. That's going to be quite slow, though... and I'm not sure how to fix that.

...OK, an ATMEGA328 has a lot less memory than I thought. Even an ATMEGA2560 is a bit cramped. Hmmm... what about a NodeMCU board? I have a couple of those... they have ESP8266s on them, which have 80k of userspace RAM if I'm reading the specs correctly. *That* ought to be plenty.

> Look up the Kaypro II computer some time.

A M328 plus some RAM plus a SD card slot runs CP/M. You can run WordStar if you find a neat 80x25 display and a keyboard.

—▷ https://www.mikrocontroller.net/articles/AVR_CP/M

Or CP/M on ESP8266, ESP32, Ar"DUE"ino or Teensy? MSGitHub has them...