In the very, very beginning, Adobe charged USD$50 for the Acrobat Reader instead of giving it away. Had it retained that strategy, the company wouldn't exist on the business desktop today. The decision to make Reader both free and freely distributable has played perhaps the single most significant role in the success of the PDF format, and of Adobe Systems itself.
Apart from being free, Adobe deliberately made Reader rugged enough to open almost any PDF file, regardless of how corrupt. It had to. Remember, the Reference gave (gives) few rules for building PDF files. Mainly, it offers just the "pdf building" vocabulary itself. When Adobe released the Reference it set itself up for a tidal wave of dubious PDFs, and not only from third parties. Modern document authoring software offers users so many possibilities that Adobe's own PDF creation software itself often doesn't know exactly what to do.
In an important sense, then, it is wrong to say that PDF is the de facto standard. "The de facto standard for PDF validation is Adobe Reader," says Appligent's CTO, Mark Gavin. "Unfortunately, Adobe Reader is not, and never was intended to be, a validation tool."
Adobe makes Reader do the software equivalent of a triple back-flip to ensure that the application will open pretty much anything it's asked to open. That is why Reader is a relatively large and resource-intensive application compared to "alternative" free PDF viewers such as Foxit. Those applications don't even try to open any heap of bytes with a .pdf extension, but Reader does.
Adobe had realized that compatibility is far more important than formal compliance. Since in the early days the company still needed to evolve the Reference, Adobe made compatibility a matter for the Reader, NOT a matter of the Reference. As a result, the company now finds itself propping up a legacy of rock-solid support for old and vague documentation. As a result, Adobe maintains a zero-revenue Reader that chugs while the rest of the industry enjoys gets a free lunch while looking forward to a free dinner -- Adobe's continued expenditure on developing new features for the PDF Reference.
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.