Suddenly having problems opening PDF files on Mac

Hi, I save my files on Dropbox and work on a Mac OS with High Sierra V 10.13.6 and over the last two weeks, many of my PDF files will not open. The message I get is “because it is either not a supported file type or because the file has been damaged (for example, it was sent as an email attachmnet and wasn't correctly decoded).” I did not have any problems opening these files before. I have checked for updates and downloaded them for Acrobat, tried to change the file name to .png and also tried to open them in Photoshop and Preview with no success. I hope you can help me.