More than ever, the Adobe "Portable Document Format" or PDF is a critical format for communication. One feature of the format is that it bridges the world of the web with the world of print very well.
PDF Tools
Surprisingly, I could not easily create a new PDF that was a combination of two other pdf documents. That's when I found the "PDF Toolkit" http://www.pdfhacks.com/pdftk/
OpenOffice does output pdf, and so does KPrinter, but the PDF Toolkit offers many more features that really should be present in a GUI program like Kpdf -- which is a viewer rather than an authoring application. At least you can install the "Service Menus" into Konqueror to get some easy right-click integration. It would be interesting to use DCOP to create a simple merge GUI.
PHP developers should look at the libraries provided there for manipulating PDF forms.
See also Format conversion and for printing any of these wiki pages as a PDF, see Collections