Error - There was an error opening this document. The file is damaged and could not be repaired.

I have been involved in IT for over 25 years. This problem has been happening more and more lately when I download a PDF file, and I am extreemly confused by the answers I have been finding. I have seen many iterations of the Reader program and I feel that Reader has been the most trusted PDF reader since the PDF format was created by Adobe. I have had so many issues with web browsers displaying PDF files correctly that my practice is to always download the PDF files to my computer and then open them with Adobe Reader and yet, this problem is happening more and more. I have cleared my Internet cache files, I have uninstalled and reinstalled the Adobe Reader program, and I have attempted to repair the Adobe Reader installation to no avail. The only thing I haven't done is to completely reload my operating system. Adobe Reader would not open several PDF files and the PDF files are from different sources. To test my theory, I uninstalled Adobe Reader and downloaded another popular reader that I have encountered in the past and the files open correctly each time (so far). So, what has happened to the program that I have used for decades?