Step 1) Convert your files to PDF.
Step 2) Merge the PDFs with PdfCopy.
Most Office formats can be converted to PDF (for free) via OpenOffice.org calls. There are probably some service sites out there that can do the same, and various commercial software packages. Vichle's link in his comment will probably do the trick.
Vichle's link also demonstrates that you probably didn't search SO first before asking your question. Naughty naughty.
Merging PDFs is fairly trivial:
Document doc = new Document();
PdfCopy copy = new PdfCopy(doc, outputStream);
doc.open();
String paths[] = getPaths();
for (int i = 0; i < paths.length; ++i) {
PdfReader reader = new PdfReader(paths[i]);
/**** The first page is ONE, not zero ****/
for (int j = 1; j <= reader.getNumberOfPages(); ++j) {
PdfImportedPage curPg = copy.getImportedPage(reader, j);
copy.addPage(curPg);
}
}
doc.close();
That's Java, but to convert to C# you really just need to switch the function names to upper case, and tweak a couple class names. It's trivial.
And supporting evidence that C# is just MS's pet Java.