I also use a clear acrylic medium called Nova Color Gel; it's a polymer. Been using it for many years and it works very well. I use it on canvas and board for all my collage. I like using the mat version because I don't like the gloss shine to it. It dries clear and you can even use it as a primer, and paint over it in acrylics and/or oils. I also use it for fabric. Is sometimes can bubble or wrap if there's not enough of it on the back of the paper (air bubbles), so you just have to make sure you spread it evenly (while it's wet) on the surface. Don't use Elmers.
For paper onto paper, you can use professional paper glue that is used in bookmaking. It is any brand of archival neutral PH adhesive. The same goes with this in spreading it evenly, but on the back only. Try not to get any on the front.
With the polymer onto to board (masonite, canvas, etc.) You can spread it on both sides.