I have a pdf that I can't copy text from.

I have tried several pdf readers: Foxit, Sumatra, PDF-XChange, mupdf  
NONE of them allows me to simply copy the text!

What could cause this?

**Edit: solution found!** This was a problem about a password protected PDF file, you can disable password protection by using this program: http://freeware-pdf-unlocker.en.softonic.com/ More info: http://superuser.com/questions/244918/how-does-a-pdf-prevent-itself-from-being-printed/244942#244942