Why can I not find the help I need

I too was having this problem and getting frustrated with the lack of good advice.  Everyone seems to assume it is the PDF that is bad, or just user error.  After just making do and getting very annoyed for the last few months (if I keep trying to open the PDF eventually it will open) I discovered something from another thread that seems to fix this permanently.  (At least for me in Windows 10)

Change the "compatibility mode" for Adobe Reader to be for Windows 7.  I changed this on the file "AcroRd32.exe" which is found in "C:\Program Files (x86)\Adobe\Acrobat Reader DC\Reader" for me.

Right click on the file, select "Properties", then the "Compatibility" tab.

Now click on "Change settings for all users"

Now you can check the box "Run this program in compatibility mode for:" and select "Windows 7"

Hope this helps someone! (Also hope this helps others not be so quick to jump to conclusions about this problem.)

More than likely, it's an issue with the file rather than Reader. In this case more specifically an issue with a font not being embedded. The other viewers aren't as picky about pdf standards as Adobe Reader is so that's why they open the file without a warning. Reader is just letting you know that the file isn't ready for prime time.

Sometimes a problem is new. Or the person who had it before never answered the questions so we couldn't get anywhere with it. Don't assume you will always find soluions; sometimes you have to engage...