Moving PDF to ISO means that Adobe Systems will submit the format to industry control, allowing third-parties to contribute to the future development of the PDF specification as equals of Adobe itself. One "interested party", one vote; something of democracy brought to software development.
Why would Adobe do this? First, the PDF Reference, which describes the guts of PDF in programming terms, is already a freely available document, and has been since 1993. The decision to "go for ISO" rests in very large part on Adobe's success with its PDF products. At the same time, Adobe's publishing the Reference in theory allowed others to make equally capable software for creating, changing or viewing PDFs. Indeed, the decision to release the PDF Reference was one of the few key moves by which PDF become the de facto standard for electronic documents in the first place.
If it hasn't always appeared to act consistently with the ideal, Adobe knows that a world awash in PDF is very likely to be a world where Adobe Systems remains a large and important company, not merely a vendor of high-end desktop software to niche markets. PDF remains key to this plan, and releasing PDF to ISO is Adobe's best possible move in taking that plan to the next level.
At the standards level, regulatory and competitive pressures blend. For many of the same reasons as Adobe, Microsoft has driven its own Standards agenda for OOXML, and it is in a maze of lobbying, legal and technical races with ODF, PDF and other Standards contenders to satisfy various industry bodies and government regulators as to how genuinely open and practical it really is.
Of course, the lowly customer just wants everything to harmonize such that (for example) an authoring document's structure is entirely preserved in the final-form version. These various standards efforts aren't structured to complement each other -- that would be far too useful. There is competition in standards, as in software, and electronic document technology is a major battleground.
At the end of the day, after the years of hard work that everyone acknowledges are required to move PDF from Adobe's PDF Reference 1.7 to a full-blown ISO Standard, developers will gain clear and reliable guideposts for their own PDF application development, and their customers will gain new confidence in their technology investments. The result will be more competition; lower prices, better products and more choices for end-product consumers. In the face of OOXML, XPS and ODF, this was a vital move for PDF, and it comes none too soon.
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.