Update
As @VCL pointed out in his comment, just exporting a list of graphics does not work since the braces and commas of the list a exported as well. Additionally, the pdf is one single page. Here is an updated approach, which takes all imported pdf-pages and inserts them into a new notebook where every page is separated by a pagebreak.
The resulting pdf has at least several pages, but content of the page is scaled and (if not turned off) the headers are printed too into the pdf.
First, we simply import the "Pages" from a file
pages =
Import["http://amath.colorado.edu/documentation/LaTeX/Symbols.pdf", "Pages"];
Now we try to make a new pdf which has the original file appended to itself. For this I join the pages together, riffle a newline-cell between them and create a new notebook which is instantly exported into a pdf-file:
Export["tmp/test.pdf",
CreateDocument[
Riffle[Join[pages,pages], Cell["", "PageBreak", PageBreakBelow -> True]
], Visible -> False]
]
This works here (MacOSX) but when you look closer at the created pdf, you see things like

while in the original document this was type-set properly

Not to forget, that the resulting pdf is much bigger than two copies of the original file would have been! Input pdf file size: 256K, Mma output: 3.2MB, gs output: 176kB.
gswin32 -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUT=merged.pdf -dBATCH 1.pdf 2.pdf 3.pdf– halirutan Oct 18 '12 at 10:09