id(obj) can and will hit the same value for different objects, if they do not exist at the same time. In CPython, its just a memory address. When that collission occurs, PdfWriter when asked to insert a page from one PdfReader object may silently confuse it with one from a since-deleted PdfReader object, thus corrupting output in not-easily-noticed ways.

Replacing dict[id(obj)] with dict[weakref(obj)] should suffice, as we do not replace entries - we only ever insert or delete.

This reverts commit 8188553.