Mladen is totally on spot here, just let me add something…
The privacy screen filter is an about .5mm thick sheet, i.e. it is quite sturdy. Since it also reduces brightness a bit you probably do not want it permanently attached to your creen anyway but only in situations where you have potentially non trust worthy people around (train, bus, airport etc.).
So there are two types of fixtures ocming with it in the package, transparent sticker strips and transparent holders. The sticker strips are used to indeed stick it to the screen long term, these are like double sided transparent sticker tape. The holders are more like “ears” that attach to the bezel frame, there are six of these in the package (IIRC). You can then just slide the screen filter behind these, between holder and screen. This holds the filter to the screen just fine and can be slid in and out at any time, just as you need it.
For the Librem5 phone this is a little different. There you do not have the space for holders so the L5 privacy screen has a special sticking surface on its back. This is less like glue but more like a nano sticking material that kind of sucks itself to the screen glass - pretty cool (feels a bit like the glue on post-it note stickers). So you can also attach and remove it many times, you just have to be very careful that no dirt accumulates on the sticky side.