The PDF does not contain vector graphics but rather an embedded image and/or the page was scanned. This means that no character/line information is available which would enable (re)conversion to a DXF document. If only parts are missing then these are often either the legend or the logo of the CAD original file of a CAD drawing which otherwise has character and line information available. As part of a crosscheck, the page can be exported with the profile: Images – Contained images. If images are exported, there is no output to DXF for the corresponding areas.