The only pdf to cdx conversion that makes sense, is potential export or recovery of graphics assets from PDF documents to CorelDRAW format, or perhaps even to previously exported .cdx files.
CorelDRAW allows the opening of PDF documents and is the tool that uses CDX format for one of the variants of its drawings, which means it's pretty much the obvious choice here. As for other software, you can certainly import PDF files to many tools, but it will be difficult to save into Corel format without Corel software.
Updated: November 7, 2023