After downloading and installing acrobat many of my pdf files are now password protected. But I never input a password and now cannot modify or combine these files. Is there a default password?

I recently started using adobe acrobat but after I installed it, I suddenly find that about 1/3 of my pdf files are password protected; I never input a password and so have no idea of what it is or how to fix it. Now I can't edit or combine those pdfs which was the whole reason I started using acrobat in the first place. Anyone encounter this problem and know a fix? How can files be password protected if I never input a password?