Folding Napkins and Pages on Shabbat

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.