Question: Is it permissible to use Post-it notes as bookmarks on Shabbos?
Answer: The Rema (OC 317:3) writes that while one is technically allowed to open temporary stitching that ties clothes together, one shouldn’t do so in front of one ignorant of hilchos Shabbos (See Beis Yosef OC 317:8; Shemiras Shabbos Kehilchasa 15:n209).
The Mishna Berura (317:21) writes that there is a machlokes as to what is considered temporary. According to the Taz, it is only considered temporary if it will be undone that day while the Levush holds that if it is sewn by a manufacturer and undone by the customer when he takes it home then that qualifies as temporary. R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 9:n55) notes that we pasken like the Levush. Thus, one may remove clothing tags on Shabbos (ibid. 15:63).
Rambam (Shabbos 10:11) and the Shulchan Aruch (OC 340:14) write that sticking two pieces of leather or paper together is a toldah of tofer, sewing. Thus, R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 5:39:2; 9:41) writes that one would only be allowed to undo threads or remove something sticky that was tied or stuck on for very temporary use. This would include anything that is supposed to remain stuck until the consumer wants to unstick it. Thus, it would seem that he would forbid using Post-it notes on Shabbos.
Nonetheless, R’ Eliezer Waldenberg (Tzitz Eliezer 16:6:4) and R’ Ovadia Yosef (Yechave Daas 6:24) disagree, writing that such sticking like the backing on plasters is designed to be temporary and removed immediately before use. The Magen Avraham (OC 340:18) writes that one may separate two pages that had accidentally gotten stuck together with wax as it wasn’t done to last. Thus, while one would not be able to stick a regular sticker onto paper on Shabbos, one would be able to use Post-it notes.
Likewise, R’ Moshe Stern (Baer Moshe 1:36; 2:29:2) demonstrates that the issur of tefira is specifically using something to connect two other things together, and wouldn’t apply to plasters, etc. (See Igros Moshe OC 2:84).
Similarly, the Aruch Hashulchan (OC 317:18) writes that the difference between tying and stitching is that knots can be untied and tied again whereas items that are sewn or glued together. As Post-it notes are so easily moved, it would seem that they do not come under this category of tofer.
R’ Yisroel Dovid Harfenes (Nishmas Shabbos 7:206) writes that especially as such notes are designed to be disposable and moved regularly, there is no prohibition in using them as bookmarks.In conclusion, one may use Post-it notes as bookmarks on Shabbos.