If you have a PDF reader that can export to RTF or other authoring formats, try that. I just verified that it mostly works, using Acrobat Pro DC to open a PDF 1.6 (PDF/A) document, Enable Edit, File » Export to » RTF, then opened in FM2020.
As David points out, PDF is not an authoring format, but there are many ways to perform content recovery for on-going maintenance & republication.
When trying to open old pdf files in FrameMaker - the thumbnail fields are empty
When importing - not opening - PDF files and they do not show something in FrameMaker, then there is no preview image in the PDF. Open the PDF in a PDF viewer to see what's in it. If output is created from such an FM file which has imported PDF, you will see the contents of the PDF also.