If you do not have access to Adobe InDesign which can of course import layered PDF documents, you have to use some of the dedicated conversion software such as the PDF2ID Enterprise, which allows users to produce IDML files from PDF sources. It works best with documents produced by InDesign or similar software.
Convert Adobe Portable document format to InDesign Markup Language document.
We have found 3 software records in our database eligible for .pdf to .idml file format conversion.
Microsoft Windows software - convert pdf to idml on Windows
Professional desktop document publishing software from Adobe
Convert from pdf file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned pdf to idml. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A converter for InDesign documents
Apple macOS / Mac OS X software - convert pdf to idml on OS X
Mac version od Adobe's popular document designing and publishing software