Take a look: Simple .NET PDF Merger
There are a couple of PDF mergers available on the Internet. But, either they are commercial products or don't support printing of the header and/or footer text, which is particularly interesting, e.g., to print the page number.
The presented PDF merger uses the open source PDF library iTextSharp to process PDF files. The sample solution also includes a tiny Windows Forms application to demonstrate the functionality.
Fortunately we have such a good thing as code project, don't we dude?