PDF files can normally be protected / encrypted in two different ways:

1. Owner password. Also called print / edit password. Anyone can open and view the document, but printing, editing and copying can be prevented. Users must provide password to enable these actions.
2. User password. Also called Open password. Document cannot be opened without providing the password.

Owner password does not actually encrypt the file and simply expects client software to respect permissions. These type of passwords can be removed quickly and easily. There are plenty or tools available that can do this. I normally use A-PDF Restrictions Remover. At $9.99 it’s really affordable and I bought it without many thoughts. They also provide 15 day trial version that seems to be fully functional. I assume they could change this at some point as many people will not see any need to pay for one-off password removal tasks.

User password does encrypt the file. This type of security can be pretty hard to crack depending on password complexity and encryption algorithm used. You can try to crack user password using brute force method, but success is not always guaranteed. Again, there are many tools available (such as PDF Password Cracker Pro). It’s a good idea to try demo version to see if the password can actually be cracked before you part with your money.