Welcome to the forum.
See
Splitting drawings on multiple layers in the Draw manual which describes how to make a layer visible or invisible. You can therefore decide which layers to export to a PDF as a "combined into one layer" PDF, but you cannot export them as "separate layers to be inserted in the PDF as separate layers in the PDF".
You could export each layer as a graphics image (or even PDF) and then use some other application to create a "PDF with layers". For example see
PDF Editor Pro which handles layers in PDF files.
You will find much useful information in the
User Guides, the
Writer, Base and Calc Tutorials and the
AOO Frequently Asked Questions. May I suggest you bookmark the pages.
Showing that a problem has been solved helps others searching so, if your problem is now solved, please view your
first post in this thread and click the Edit button (top right in the post) and add [Solved] in front of the subject.