Background
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:

• "Open"-Password (Password protect the opening of a PDF Document)
• "Owner"-Password (enable/disable various Document Security- and Access-Settings)
• Disable "printing" of the PDF Document or restict to low resolution printouts
• Disable copying of any Docment contents
• Disable any changes within the Document and guarantee the authenticity
• Locks any PDF Document immediatly
• Works with any PDF Document (not only "Acrobat-made" ones)

Securit Settings
There are three pre-defined Setups for encrypting your Documents:

Basic: just apply an "Open"-Password to your PDF Document:

• The Document will be encryted using strong 128-Bit Adobe Acrobat Encryption
• The recipient of the Document has to enter the "Open"-Password" to view the PDF
• The encrypted Document will be compatible with Acrobat Reader 5.0 or later
• All Meta Data within your Document will be encrypted
• Any copying of any content of your Document will be disabled
• Printing of the document will be allowed in high resolution
• The Security Settings of the encrypted Document are not changeable

Advanced: apply an "Open" and/or an "Owner"-Password to your PDF Document:

• The Document will be encryted using strong 128-Bit Adobe Acrobat Encryption)
• If you set an "Open" Password the recipient of the Document has to enter the "Open"-Password" to view the PDF
• The encrypted Document will be compatible with Acrobat Reader 5.0 or later
• All Meta Data within your Document will be encrypted
• Any copying of any content of your Document will be disabled
• Printing of the document will be allowed in high resolution
• The Security Settings of the encrypted Document are changeable after opening the Document using the "Owner"-Password

Pro: Set any Adobe Acrobat supported Security and Access Option individually:

• Compatiblity (Acrobat 3.0, 5.0, 6.0 or later)
• Encryption Level: 40-Bit or 128 Bit
• Select Document Components to encrypt
• Set an "Open"-Password
• Set an "Owner"-Password
• Enable/Disable printing (none, Low resolution only, High resolution)
• Enable/Disable changes (various settings, depending of selected Acrobat compatiblity)
• Enable/Disable copying of Document contents
• Enable/Disable text access for screen reader devices for the visually impaired