PDF files are self-contained documents with internal cross referencing, object IDs and so on. You can't just run them together. You either need a PDF reading/writing library or, if the PDFs are simple and you can follow the format, you need to read in all the objects, renumber the IDs of the objects, update the PageCatalog, and fix the cross reference table.
See the PDF spec (
here[
^] is the spec for v1.4) for more details if you want to go down the route of reading the files yourself.
This is really not a job for a stored procedure, there is serious coding involved.