Protecting a PDF
From Adobe Acrobat's Help file
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Security
You can use passwords to restrict users from opening, printing, and editing PDFs. You can use a certificate to encrypt PDFs so that only an approved list of users can open them. If you want to save security settings for later use, you can create a security policy that stores security settings.
By adding security to documents, you can limit viewing, editing, printing, and other options to only the specified users. You can choose if you want the users to have the required password, a digital ID, or access to Adobe LiveCycle Rights Management.
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Security methods
Acrobat provides different security methods for specifying document encryption and permission settings. You can encrypt all or part of a document and limit user actions. For example, you can allow users only to fill in form fields or prevent them from printing a PDF.
Each security method offers a different set of benefits. However, they all allow you to specify encryption algorithms, select the document components to encrypt, and set permissions for different users. Use the Document Properties dialog box to choose one of the following security methods:
§ Password security provides a simple way to share documents among users when sharing passwords is possible or when backward compatibility is required. Password policies do not require you to specify document recipients.
§ Certificate security provides a high level of security, eliminates the need for password sharing, and allows assigning different permissions to different users. Also, you can verify and manage Individual user identities.
§ Adobe LiveCycle Rights Management policies are stored on a server, and users must have access to the server to use them. To create these policies, you specify the document recipients from a list on Adobe LiveCycle Rights Management.
Security policies
After choosing the security method, you can save the settings as a policy. Policies save time while ensuring a more consistently secure workflow. They allow you to create a reusable library of pre-configured security methods.
Importing and exporting security settings
You can easily share some or all of your security settings with others by exporting and importing the settings. In enterprise environments, you can save the security settings on a server, and then load them by specifying the URL for the server. You can also use the exported settings to back up and restore your settings, as needed.
Security envelopes
You can add security to one or more documents by embedding them in an encrypted envelope, called a security envelope. Envelopes are PDF files with attachments. This method is especially useful if you want to send a few secure file attachments without encrypting the files. You can embed the documents as file attachments in a security envelope, encrypt the security envelope, and send it to the recipients. When the recipients open the envelope, they can extract the file attachments and save them to disk. The saved files are identical to the original file attachments and are no longer encrypted when saved.
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In addition you can (with Arobat 9) password protect the document up to 256-bit AES standard. Earlier versions up to version 6 (I think) only allowed protection up to 128-bit standard.
Nuance's PDF Converter 6 does the same (256 and 128-bit protection) etc and is considerably cheaper.