As far as I know, the .iso disc image files can't have the password protection feature, it's something weird, too. Probably what you've downloaded is a protected archive with the .iso file inside. You could try a password brute-force scan with a passwords recovery program like
this, it's only needed lot of time.
Sounds like you've downloaded one of those pirated programs where the "password" is put in a text file, but in it there's the link to a survey website which asks for cell phone numbers, and if you're afraid to complete it, then maybe you'll download another text file with the password written...