Error opening the documents

I have a two files as a PDF format. I cannot open them because the following error: There was an error opening this document. There was problem reading this document.
Any suggestions how to fix PDF files?

What are you using to open them?
Did you compile them to pdf?
Can you compile them to pdf again?
What media are they on?

Read info here: https://onlinefile.repair/en/pdf.html
Also refer to the following Adobe Article and check if this helps: https://forums.adobe.com/thread/392984