Medieval lords had castle accounts, right? On what were these written? Did they call them paper, or parchment? Were they made of dried skins, linen, paper?
During the 8th century, Chinese papermaking spread to the Islamic world, where pulp mills and paper mills were used for papermaking and money making. By the 11th century, papermaking was brought to Europe. It was common enough by the 13th century for a decree from Holy Roman Emperor Frederick II in 1221 to declare all official documents written on paper to be invalid! https://paper.gatech.edu/invention-paper-0
“Medieval paper was made from linen rags. It is much stronger and more durable than modern wood-pulp paper, and fifteenth-century scribes were wrong if they believed that it would not survive. Rag paper is manufactured as follows. White rags are sorted and washed thoroughly in a tub pierced with drainage holes and they are then allowed to ferment for four or five days. Then the wet disintegrating pieces are cut into scraps and beaten for some hours in clean running water, left to fester for a week, beaten again, and so on, several times over, until the mixture disintegrates into a runny water-logged pulp. It is then tipped into a huge vat. A wire frame is scooped into the vat, picking up a film of wet fibres, and it is shaken free of drips and emptied onto a sheet of felt. Another layer of felt is laid over it. As the soggy sheets emerge and are tipped out, they are stacked in a pile of multiple sandwiches of interleaved felt and paper. Then the stack is squeezed in a press to remove excess water and the damp paper can be taken out and hung up to dry. When ready, the sheet is ‘sized’ by lowering it into an animal glue made from boiling scraps of vellum or other offcuts. The size makes the paper less absorbent and allows it to take ink without running. The sheets may have to be pressed again to make them completely flat. Sometimes, especially in north-east Italy (doubtless under the influence of Islamic paper manufacture) the paper was polished with a smooth stone to give it a luxurious sheen.” http://linux2.hit.uib.no/non/echt/budapest/ManMan/paper.html
Paper was used in Wales, certainly, but it was viewed as less sturdy than parchment and also less high status. A surviving manuscript (from the thirteenth century) is in the National Library of Wales. It was a ‘pocket’ book of the laws of Hywel Dda (from the 10th century), designed for lawyers to carry around in their scrip, rather than left on a library shelf. Another manuscript, made mostly of paper and thus not very ‘high status’ is the Jesus College 141 manuscript of legends of Wales, written in Welsh. https://research.bangor.ac.uk/portal/files/20577287/null
On the other hand, parchment was something different, and also used, though it was a more precious substance than paper. It was: “a writing support material that derives its name from Pergamon (Bergama in modern Turkey), an early production centre. The term is often used generically to denote animal skin prepared to receive writing, although it is more correctly applied only to sheep and goat skin, with the term vellum reserved for calf skin. Uterine vellum, the skin of stillborn or very young calves, is characterised by its small size and particularly fine, white appearance; however, it was rarely used. To produce parchment or vellum, the animal skins were defleshed in a bath of lime, stretched on a frame, and scraped with a lunellum while damp. They could then be treated with pumice, whitened with a substance such as chalk, and cut to size. Differences in preparation technique seem to have occasioned greater diversity in appearance than did the type of skin used. Parchment supplanted papyrus as the most popular writing support material in the fourth century, although it was known earlier. Parchment was itself largely replaced by paper in the sixteenth century (with the rise of printing), but remained in use for certain high-grade books. See also flesh side and hair side.” http://linux2.hit.uib.no/non/echt/budapest/ManMan/glossary.html#parchment
Illuminated manuscripts of whatever nature were exclusively on parchment until the late Middle Ages: “The majority of surviving manuscripts are from the Middle Ages, although many illuminated manuscripts survive from the Renaissance, along with a very limited number from Late Antiquity. The majority of these manuscripts are of a religious nature. However, especially from the 13th century onward, an increasing number of secular texts were illuminated. Most illuminated manuscripts were created as codices, which had superseded scrolls. A very few illuminated manuscript fragments survive on papyrus, which does not last nearly as long as vellum or parchment. Most medieval manuscripts, illuminated or not, were written on parchment (most commonly of calf, sheep, or goat skin), but most manuscripts important enough to illuminate were written on the best quality of parchment, called vellum.
Beginning in the late Middle Ages manuscripts began to be produced on paper. Very early printed books were sometimes produced with spaces left for rubrics and miniatures, or were given illuminated initials, or decorations in the margin, but the introduction of printing rapidly led to the decline of illumination. Illuminated manuscripts continued to be produced in the early 16th century, but in much smaller numbers, mostly for the very wealthy.