Renaming layer structure

Good morning all,

As i was making a PDF file with multiple PDFs in it, I bumped on to something i cant figure out.

When i need to turn on/off layers in the PDF, all the layers i plot appear.

The only thing that doesnt appear is the name of the PDF page where the layers are placed.

As you can see on the attached file there are 5 pages in the PDF.

I opened the first one and all the layers are visible.

But the name of this first page is existing out of random letters and numbers ( f1a88 etc.etc.)  instead of how i named this page.

I am not able to double click or right click on it to rename this.

Is there any posibility to rename these 'codes' back to how i named the pages itself?

thanks in advance,