Reader file won't open and I've tried everything

[#1]

Adobe Acrobat Reader could not open 'XXX name of file' because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded).

MacAir - MacOS 10.13.6

I have uninstalled every version of Adobe, reinstalled Reader to version 2019.021.20061.

Some PDFs open, some don't. 

I scanned documents thru an HP Envy 4500.  I do that almost literally daily.  No problem till now so I'm really not feeling that it's the scanner.

Help!

[#2]

Adobe Acrobat Reader could not open 'XXX name of file' because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded).

MacAir - MacOS 10.13.6

I have uninstalled every version of Adobe, reinstalled Reader to version 2019.021.20061.

Some PDFs open, some don't. 

I scanned documents thru an HP Envy 4500.  I do that almost literally daily.  No problem till now so I'm really not feeling that it's the scanner.

Help!