to fold and pile up; to fold something and stack the folds (transitive).
古い新聞紙を折り重ねて段ボールの隙間に詰めた。
I folded and stacked old newspapers and stuffed them into gaps in the cardboard.
布を何度も折り重ねて簡単なマットを作った。
I folded the cloth repeatedly and made a simple mat.
書類は折り重ねると厚くなるので、ファイルに入れて保管したほうがいい。
If you fold and stack documents they become thick, so it's better to store them in a file.