A PDF should be able to open in Books app, which is included with iPhones and iPads. If Books fails to open it, then it could be a Books bug, a bad PDF, or a locked PDF. Did you create the PDF? If so, which app was used to create it?
I'm a little confused on how you are trying to open the PDF. What app are you trying to open it with? If all else fails, email the PDF to your iPad, or transfer it via the File app, then try to open it in Books.