most 15th-century paper is of a very high quality, as is the paper used for the Gutenberg Bible.
later the quality of paper declined - most disastrously in the 19th century when paper-makers began using wood pulp. the paper used in the Gutenberg Bible was imported from caselle in piedmont, northern italy being one of the most important centres for paper-making in the 15th century.
it can be identified because its watermarks. about 70% of the paper has the watermark of an ox head, 20% show a bunch of grapes (in two versions),
its size is known as royal folio, already at that time a fairly standard size of paper, each sheet measuring about 430 x 620 mm, before being folded.
how is this paper made? take a look at this video.