Unable to open old pdf files using FrameMaker

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.

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.

FrameMaker doesn't "open" PDF files. You can import it as a graphic but cannot edit it.

Can you give more info as to your goal?