If I save a PDF from a original Illustrator file with layers in Illustrator 25.6 as a Acrobat 6 (PDF 1.5) with Illustrator editing capabilities turned on, I cann´t see the layers after oppening the PDF in Illustrator 25.4.1.
That is correct. Layers in a PDF are not the kind of layers you see in an Illustrator file.
"PDF Layers" are used to show/hide groups of content and are named Optional Content Groups.
And when opening in an older version, you will open the PDF part instead of the layered Illustrator part of the file.
Thank you for this answer, but could it be that this was possible in older Illustrator versions and do not function with the newer one? And one more question does this function between the same Illustrator versions? I want to send this PDF to another user and he wants to work with this PDF in the same Illustrator version and need the layers.
It was not possible in older AI versions. Illustrator layers are only visible in Illustrator.
If he wants to work in Illustrator, why would you send a PDF instead of the Illustrator file?
Now I`m a little bit confused. I get PDF with layers created in Adobe Illustrator 25.4.1 and if I open this PDF in Adobe Illustrator 25.6, I get the layers and can work with it!
If the PDF with layers created in Adobe Illustrator 25.4.1 had the Preserve Illustrator Editing Capabilities checked when saving, you will see the layers in newer or compatible versrions.