How do I stop index's going missing when editing pdf?

Hi there

I have created index's for all my pdf textbooks that I use for uni.

Now and then I'll edit the table of contents, or crop something, or edit some text slightly.

Each time I do this I lose the index for quick searching, and I have to re-index all over again.

Some of the textbooks are huge so this is a tedious process to re-index each time.

Is there anyway I can keep the index?