Altering color in a PDF falls to a few trusty tools. PitStop Professional is the best tool for preflight, correction and editing PDF text and graphics. It currently supports nearly all of the PDF color spaces, and comes in handy for global color changes.
Images are best handled by ARTS PDF ImageWorks which provides numerous tools for editing image data and managing whole images, such as, image substitution or extraction. Image size is the single biggest difference between a document for print and one for the web or CD. For these applications, it is not necessary to convert CMYK to RGB to be web-ready. CMYK works quite nicely when viewing "web-ready" PDFs in Acrobat.
There are three cases for trapping page data: before it is a PDF, when it is a PDF, or after it is a PDF. Before it is a PDF comes in two basic flavors; composite color traps, or pre-separated trapping from applications where an Acrobat plug-in, Seps2Comp, has recombined the separated PDF. The objective is for all data to comply with the same standards, in this case, composite color. The second case is trapping a PDF. Ideally, one wants to trap directly in PDF. Currently the only product that can do this is SuperTrap from Heidelberg. Part way to the third case are tools that allow you to set the trap parameters and page zones for in-RIP PostScript trapping. The other variations of the third case are downstream PostScript trapping applications. These applications should be PostScript 3 fluent to support all the various PDF and PostScript color spaces. One example is Creos TrapWise.
Another curious state exists when you try to edit a PDF image or graphic in Photoshop or Illustrator. What happens when you edit a color managed document on a workstation where color management is turned "Off". Are you editing the color managed, or unmanaged data? Alternatively, even if color management is "On" in the workstation, you may want to verify that the appropriate profile is in use in both Acrobat and the editing application, otherwise your color adjustments will be off.
When Acrobat 4 was announced, it was hailed as ready for graphic arts applications because it now supported ICC color. Ironically, it was Acrobat 3 that was ready for graphic arts applications because it supported CMYK and spot colors.
Moreover, there is a wide spread misconception about what the ICC color support in Acrobat is. Many people assume that you can attach an output device profile and everything will come out okay at the printer. If you were to do this you would be making your PDF document device dependent. Adobe correctly did not adopt this approach. For the ICCBased color space, Acrobat supports the "A to B tags" (scanner and monitor profiles), and not the "B to A tags" (printer profiles). In other words, Acrobat is supporting the conversion of input data into common color space coordinates that can be understood by all downstream processes and printers. Because it is the responsibility of the output device to render the colors correctly, PDF documents remain portable.
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.