Can't print a downloaded PDF

I download PDF files from a website onto my MacBook several times per week to print off. suddenly yesterday the downloaded files wouldn't print, the print preview showed a blank white page, and on one file a box popped up that said "password required". I can view the files in safari and when I saved one file to my desktop, it was blank. I have never password-protected any PDF on my computer. Does anyone know what could have happened and how can I fix it?  thanks