Is one permitted on Shabbos to use post-it notes or adhesive page flags to mark content in a book for future reference after Shabbos? Would there be an issue with the adhesive or leaving the page flags in the book for an extended period of time (but eventually removing them)?
After discussing this question with poskim over the years I have found there are those that are matir. They all base their heter on the Magen Avraham in Siman 340 Sif-Katan 18. He writes there that if papers (dafim) were stuck together through shaveh (wax) then they may be "opened" (taken apart I suppose) on Shabbos. See there in the Shulchan Aruch (Sif 14 and the M"A Sif-Katan 18) for the exact context and more on what he writes. Whether the case of the post-it notes or the sticky things used for bookmarks is similar to what the M"A says, this is none the less what I have heard from poskim. Another point added in is usually the sticky things used to keep place in seforim are used for long periods taking away from the "adhesiveness" -- thus perhaps more of a reason to be makil (again what I've heard from poskim.)
Among these poskim is my HaRav Shammai Gross Shlit"a (bal machaber ShuT Shevat HaKahasi and a well known Posek in Eretz Yisrael.)
In Shemirath Shabbath Kehilchatha 16:28 it says:
All games involving cutting or sticking, whether with glue or with adhesive tape, are prohibited.
There is no further information listed, so couldn't say if there was an exception for temporary adhesive.
It makes me wonder about the plastic figures that stick to glass if you wet them - they are not really adhesive, but they do stick.
Photographs may be inserted into an album, so long as
b. they are not stuck into the album (....) and
c. the album is not of the type to which photographs adhere of their own accord when inserted.
In Shemirath Shabbath Kehilchatha 15:87 he writes that diapers where are affixed with adhesive tape ("scotch") are preferable on Shabbos or Yom Tov, because there is no concern about them at all.
See also, for a direct discussion of Post-it's: