The next time you travel by plane, notice that the pilots will get on the plane quite a bit before you do. That's not because they like their cramped cockpit so much but because they have a whole long list of preflight checks to run. By the time you get on the plane, they already know it's in good working condition...
The term preflight was derived from that process; before you make an expensive plate and start making a gazillion copies, you want to make sure the file you have is actually correct and will print the right result.
Preflight essentially acts like a safety net. It takes some extra time to go through the preflight step (you'll see later that it is perfectly possible to integrate the extra processing step seamlessly in your workflow) but consistently preflighting every PDF file that comes into or leaves your shop can save you lots of hassle.
One of the most common questions people ask when they start learning about preflight is where in the workflow it should happen. Does the designer have to preflight his QuarkXPress or Adobe InDesign file, or does he have to preflight the PDF generated from those native application files? Or is preflighting something that needs to happen when the publisher or printer receives the PDF file? There are two different factors to take into account when answering these questions.
On the one hand, look at all steps in your workflow. Whenever you are not certain about the file you receive, you have to preflight. If you just assume the previous step in the workflow yielded correct results, you open yourself up for trouble. The only exception to this rule is when you can use technology such as Enfocus Certified PDF to take away that uncertainty; if you can't guarantee the quality of files coming in, preflight them.
On the other hand, I recommend that you preflight as early as possible. If you have the opportunity to preflight the native QuarkXPress or Adobe InDesign file, do so. Detecting problems there gives you a very easy and quick way to fix them. But as a designer, you should also preflight your PDF files before you send them to your printer or publisher. Creating that PDF file is an extra and complicated step; preflighting the PDF file guarantees that it was successful. If you send out a bad PDF file it will bounce back to you and cause delays. It might even have to be fixed further down the workflow resulting in extra costs or uncertainty over the final result.
I make it sound as if you will be doing a lot of preflighting... and you will be. If you're worried about that just consider how little time it will take you to preflight in your workflow and compare it to how much time and money it will cost you if an error ends up on press because you didn't preflight.
People are always hesitant to add extra steps to their workflow, but there are few processes that have such an enormous return on investment as properly implemented preflighting.
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.