Adobe has an agreement with ISO that it can post the standard for free provided it isn’t the “official” ISO version. So the running headings and footings have been changed and the introductory pages are different, but the technical chapters are identical including page and section numbering. If you need an official version please pay for it at the ISO site. This is one source of income for this important standards organization.
Adobe should be commended for continuing to enable the PDF specification to be downloaded for free by all developers who develop PDF software.
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.