Welcome Serene,
Adobe is just such a pain sometimes I never use it. First you can download Foxit the free version does almost everything the paid version of Adobe does. It's what I use in Linux, Win 7, Win 8 & Win 10.
Here is a link:
-reader/
After you download that and install it see if it will open the .pdf files? If it does then the .pdf is fine and Adobe is the problem. If they still do not open then the problem is with the pdf file.
If the problem is with Adobe:
Un-install use something (Maybe Ccleaner) to clean the registry. BACK-UP THE REGISTRY FIRST. Then if you know how to open your AppData folders? Open the AppData folders and delete everything that says Adobe (ONLY IF THE ONLY ABODE YOU'RE USING IS THE READER). Then go to C:\Program Files and delete Adobe again, then C:\Program Files (x86) and delete anything Adobe. (ONLY IF THE ONLY ABODE YOU'RE USING IS THE READER)
If the problem is the pdf file:
Download the files to a new folder on C drive. Make a folder on C drive "Adobe" download the files to that folder and see if they open. If you're using Windows 8.1, 10 or 11 in some cases it takes your permissions away for things downloaded to the Desktop, Downloads folder or Documents folder.
Let me know?
And could you post which OS you're using?