FyTek's PDF Secure allows you to encrypt (using standard 40-bit or 128-bit supported by Acrobat Reader 5.0 and up) existing PDFs.
You may also supply password protection and lock down interactive fields. In addition, you may also set the document title, subject, author, creator and producer.
PDF Un-Secure, a free download, may be used by individuals you want to
allow full access to an encrypted PDF at some point. They will be able to
remove security with this product only when you provide them with the
owner password you encrypted the PDF with.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.