Perhaps the most intuitive way to annotate a PDF document is to add an electronic sticky note. While physical sticky notes are useful with hard-copy documents, electronic sticky notes are more efficient and provide a natural and familiar means by which to add and share comments on PDF documents without obscuring their content.
Here's how to add a sticky note in Acrobat:
Select the Note Tool from the Review & Comment menu.
Click on the page to stick down your new note.
Type your annotation and then click outside the note to finish editing.
Simple, huh? Perhaps best of all, you needn't worry about having an electronic sticky note fall off, as they are saved with the PDF document: where it goes, they go.
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.