In the West (the context for my answer), mass literacy at all levels of society is actually an incredibly modern phenomenon. One has to think no farther than that in the U.S., slave owners in general prohibited their slaves from learning to read and more particularly to write.^1 And while many children, particularly in rural areas, would surely have learned to read either from their parents or in informal group settings, an entrenched and widespread public primary education system in the U.S. is really a post-Civil War, even a late 19th century development.

That said, scholars of literacy typically distinguish between individual literacy and the idea of a literate society: a society that trusts in, depends on, and runs on written records. This is not to say a pre-literate society doesn't have readers and writers, just that recourse is not made to documents in the same way or with the same frequency as a literate society. In a literate society, information is systematized and organized in a way it can't be in a society governed by oral tradition.

People think differently in a literate versus pre-literate society. An interesting example of this is how books gain tables of contents and then, in the late 13th century, indexes. People are starting to see books as sources of information to look up, not just ideas to have memorized or things to meditate on. Illiterate peasants wanting to transfer land to each other hire notaries to draft charters, which they would not have done earlier--they trust written records in a way they did not earlier; written records are a legal necessity in a way they were previously not. I think this is similar to what OP is getting at with today's "tech literate" world.

In the west, we see the transition from a pre-literate to a literate society in the later Middle Ages--a time when, in fact, very few people learned to read at all, in any language. To be clear, literatus (literate) in the Middle Ages originally meant something very specific: ability to read Latin, the official language of governments and the Church as of ~1200 or so. By that point, the idea that the local vernaculars (Middle German, Old French, etc) had a written component was being established, but it did not carry the same prestige or importance as Latin. At this time, Latin literacy was limited basically to clergy, which would include the "clerks" working in early, developing government bureaucracies (as well as cloistered nuns, although Latin will decline in importance for them throughout the late Middle Ages even if it never goes away).

Simply by the nature of, well, written records, we aren't going to have firsthand accounts by people illiterate in both Latin and their vernacular scoffing at the lack of necessity. There is ample reason, of course, to distrust literate clerics' claims about illiterate people. Nevertheless, we can see a very interesting stigma develop. According to Michael Clanchy, From Memory to Written Record (one of THE foundational books on medieval literacy), "literatus" actually became synonymous with "clericus"--clergy!--and "illiteratus" with laicus, lay. Being Latin-literate actually afforded someone automatically more prestige in society, a higher status with certain privileges attached.

An interesting, kind of ambivalent confirmation of this comes from religious women writers in the 13th-16th century who were vernacular-literate but not Latin-literate. As women writing about religion in an era when women were not allowed to write about religion, their use of the vernacular was actually an even bigger obstacle than we might think: Church leaders were especially concerned that the laici reading vernacular writings would be spiritually corrupted by them, lacking the religious/Latin education to truly understand religious teachings firsthand. So it is interesting to watch women, who already face an uphill battle in the quest to write publicly about religion, justify their use of the vernacular or lack of Latin literacy as well. Even the rich, clearly educated ones do not seek out further Latin education: they continue to read and write in the vernacular.

Of course, these women ultimately ended up on the "right" side of history: by the 15th century, while official business remained largely Latinate, even civic records were starting to be in the vernacular, and there is quite a late medieval tradition of even vernacular Bibles.

So what can we say about truly illiterate or uneducated people? Well, this is still a time when lack of literacy (Latin or vernacular) was incredibly widespread, particularly among the rural peasantry that continued to comprise the vast majority of society. Nevertheless, urban literacy has been guesstimated (GUESStimated, seriously) in the range of 30-50%, with gentry-type writers starting to claim that reading and writing were essential skills to run a city household and business. And in Germany, here as with the landed nobility we see a very interesting stereotype develop: (vernacular) literacy is that thing women do, men have their own pastimes.

My understanding is that actual illiteracy among German noblemen and male urban bourgeoisie was not as high as contemporary sources (and even older scholarship) have claimed. In fact, even Latin (!) grammar schools for urban male middle/upper class children were a fixture in late medieval German cities. Nevertheless, it's quite striking to read writers like Hans Folz in Nuremberg circa 1480:

Any poor man who wants to get married / had better learn to read and write / immediately! If you can do it / it will make many things so much easier

Folz is writing, as he so often does, about how all these pernicious wives are taking over society and ruling their husbands and the world is upside down. Crucially, he identifies their literacy--and their husbands' stubborn illiteracy--as a major reason this turmoil is even possible.

Folz's poetry is good evidence here because it was originally performed orally, in public (scholars have speculated the written copies were sold at his performances, in fact--dude bought and operated a printing press SOLELY to publish and publicize his own poetry. That takes all the chutzpah and a WHOLE lot of money). Any actual illiterate men among his audience would have gotten the message that illiteracy isn't a joke, it's the thing that is upsetting society by allowing women to 'wear the pants' (cf another Folz poem, for real). Meanwhile, the literate men in his audience could have seen this as an in-joke or reassurance: YOU can read, YOU are not part of the problem.

So overall, we can see interesting types of both stigma attached to levels of illiteracy, and resistance to the need for different levels of literacy in a transitional society like the later Middle Ages.

~~

^1 Reading and writing have historically been two separate skills. For most of the early modern era up to the entrenchment of public education, we can talk about a distinction between "literacy for Bible reading" and functional or liberating literacy, which typically involves the ability to write. It goes back to the idea that religious reading for personal edification is a spiritual practice, but writing and being able to teach others about religion is a dangerous thing with the potential to upset society in the wrong hands--whether this is medieval women or 19th century slaves.

I live in Sweden...
Are there historical documents portraying illiterates as something negative?

Yes, there are. And geographically very close to you, even if distant in time.

For example, already in the late 17th and 18th century Sweden and Finland, which was part of Sweden for six centuries until 1809, being illiterate was something very negative because of the laws and policies.

In the 1686 Church Law by the king Charles XI it became mandatory for everyone to attend the Sunday sermons, and it became mandatory for everyone to be able to read the catechism. A person who could not read could not get church confirmation, no holy communion, and no permission to marry, effectively rendering an illiterate person an outsider.

Vicars of every parish conducted annual interrogations of religious knowledge and reading abilities in every household including peasantry, commoners and servants. And not being able to read was seen as very negative and also had its consequences mandated by the law. As a result people started to learn to read - already at young age - and literacy rates became high.

Just for one example, here's an account by Scottish John Patterson who was travelling in Sweden 1807-8 (Patterson, J., The Book of Every Land, London 1858, pp. 51-52) [italics mine]:

From Malmoe I paid a visit to my friend, Dr. Hylander, in Lund, made the acquaintance of the bishop and some of the professors, and enlarged my knowledge of Sweden. As Dr. Hylander had a parish not far from Malmoe, I one day went with him to attend an exam ination of his parishioners. It was held in a peasant’s house, in a large hall, where a goodly number were collected. The people, old and young, answered the questions put to them readily in general; those who were deficient in their knowledge were severely dealt with, and exhorted to be more diligent. On the whole the exercise was calculated to be useful. It was a pleasing circumstance that all could read. Indeed, this may be affirmed of the inhabitants of all the north ern Protestant Kingdoms; you seldom meet one above ten or twelve who cannot read, and the most of them write their own language; yet at the time now referred to, there was nothing like what we have in Scotland, a provision for the education of the people by means of parochial schools. The parents were the teachers of their children, till they reached the age of fourteen or thereabouts, when they attended the pastor or his assistant, to be prepared for confirmation and being admitted to the Lord’s Supper. And as no person can be confirmed till he can read and repeat his catechism, or, until confirmed, can give his oath in a court of justice, or get married, a great disgrace is attached to not being able to read; indeed, one who can not read is nobody in the eye of the law. This state of things has its advantages, as far as education is concerned; but, alas! it has its dis advantages, as it admits all to the enjoyment of religious privileges, and thereby tends to make a nation of religious formalists.

Even as his account is from a later period, and he could of course exaggerate a bit, there is no doubt that being illiterate had a very negative stigma, and that it rendered a person an outsider as per the Church Law in place already in the late 17th century. The Church Law of 1686 remained in effect in Sweden until 1888 and in Finland until 1869.

Additional sources:
Understanding Literacy in its Historical Contexts - Socio-Cultural History and the Legacy of Egil Johansson; ed. H.J. Graff, A. Mackinnon, B. Sandin, I. Winchester; Nordic Academic Press, 2009

National Literacy Campaigns - Historical and Comparative Perspectives; ed. R.F. Arnove, H.J. Graff, Springer, 1987