Help please! I can't open old PDF's

++adding on to the suggestion given above.

You mentioned having the new computer. Is it a fresh install of Acrobat, or did you transfer the setup from the old machine to the new one?

If it was transferred from one machine to another. Uninstall the Acrobat and download it fresh: 

If this error persists only when opening the Dropbox file, then disable the Dropbox account and reenable it once. Login to your Dropbox online account and check whether you can access those files directly.

Download the files first locally and then try to open them using Acrobat. Does it open fine?

https://helpx.adobe.com/acrobat/kb/acrobat-dc-downloads.html

Thanks,

Akanchha

Are you opening the file from Dropbox, or did you download it to your computer first?

Try opening the file in a plain-text editor, like Notepad. What does the first line of it say?

Can you share a sample file with us?