Maran rules in Siman 302 that one is not allowed to fold garments on Shabbat, unless he does so against the crease. However, the Poskim permit folding the page of a book to mark the place. This is not considered part of the Gezerah. Furthermore, it is permitted to fold a napkin with a crease, but not to create a special shape (Rabbi Shlomo Zalman Auerbach, Jerusalem, 1910-1995). Also, plastic and rubber materials are not subject to the prohibition of folding.