I want to protect my PDF with a strong password.
But I heard that it can be cracked.
Is it true that strongly-password-protected PDF can be cracked?
Edit: This is my real scenario. I want to sell my ebooks on the web. Recently there are many sites that shares ebooks. I don't want it to happen so I need a password-protected scheme applied to my ebooks. I hope buyers cannot remove and modify the contents; the integrity must be protected.