Adobe Reader Touch and DC can't open .pdf BUT a browser can

Hello, when using Adobe Reader Touch 3.1 Build 87675 on Windows 10 and Acrobat Reader 2018.011.20038 on a MAC a whole host of .pdf files refuse to open with the "The document cannot be opened because it is not a valid PDF document"

YET if I right click the document in Windows Explorer or Finder and select to open with Chrome, Edge, Safari, or Preview, the file quite obligingly opens and looks just fine.

What is this about?  What can I do to fix the file, or is there a problem with the reader(s)?

Thank you.