There is free software called PrimoPDF that has the ability to turn documents into PDFs, and also to then merge multiple documents into one file. To get it, go here:
After installing, an option will appear among your printers called PrimoPDF. Even though it's set up as a printer, it doesn't actually print, in the sense of a hardcopy. When you select it as a printer option, it turns whatever document you're "printing" into a PDF file. I realize that the documents you have are already PDF files, but that is OK. Select the first one and "print" it using PrimoPDF as the "printer", then you'll be prompted for a file name - you'll want to enter something like "2010 990 Tax Form". Then for the second one, do the same thing, and enter the same name - PrimoPDF will detect that the file already exists, and will give you options to either Append or Overwrite, and you'll choose Append. So at that point the 2 documents will now be in one file.
IMPORTANT NOTE: Don't choose overwrite or you will lose your original document.