combining layers adobe PDF

Hi everyone,

I would like to combine multiple layered PDF into a single layered PDF.

These are architectural drawings. The layers are the same for each of them (walls, doors, floor, etc.). I had to export them separately because of the scale.

The problem is that when combined, the layers are not. The left-hand column shows this:

plan-50.pdf

. doors

. wall

. floor

plan-100.pdf

. doors

. wall

. floor

And I would like to have this:

. doors

. wall

. floor

Is that possible ?

Thank you