Free Online PDF Document Encryption
A PDF document can be encrypted to protect its contents from unauthorized access. Encryption applies to all 'user-content' in the document’s PDF file.
PDF’s standard security handler allows access permissions and up to two passwords to be specified for a document: an owner password and a user password. An application’s decision to encrypt a document is based on whether the user creating the document specifies any passwords or access restrictions. If so, the document is encrypted, and the permissions and information required to validate the passwords are stored in the encryption dictionary.
If a user attempts to open an encrypted document that has a user password, the application prompts for a password. Correctly supplying either password enables the user to open the document, decrypt it, and display it on the screen. If the document does not have a user password, no password is requested; the application can simply open, decrypt, and display the document. Whether additional operations are allowed on a decrypted document depends on which password (if any) was supplied when the document was opened and on any access restrictions that were specified when the document was created.
Opening the document with the correct owner password (assuming it is not the same as the user password) allows full (owner) access to the document:
This unlimited access includes the ability to change the document’s passwords and access permissions.
Opening the document with the correct user password (or opening a document that does not have a user password) allows additional operations to be performed according to the user access permissions specified in the document’s encryption dictionary.
Our free online solution
Our Service allows you to encrypt your exisiting PDF Documents online, supporting all Acrobat PDF featuers: Securit Settings
There are three pre-defined Setups for encrypting your Documents:

Basic: just apply an "Open"-Password to your PDF Document: Advanced: apply an "Open" and/or an "Owner"-Password to your PDF Document: Pro: Set any Adobe Acrobat supported Security and Access Option individually: