Adding pages to existing pdf file

You could use pdftk from the commandline:

pdftk A=bigpdf.pdf B=insert.pdf cat A1-180 B A181-end output output.pdf

Or if you want a GUI you could use PDFsam (PDF split and merge), PDFjam or jPDFTweak. (PDF Shuffler has already been mentioned.)

PDF-Shuffler

PDF Shuffler is an ideal application for this. It allows you to import several PDF files and rearrange the pages. You can also delete the pages you don't wish to keep in the output PDF.

PdfMod works well if you prefer to use a GUI. Install it by typing:

sudo apt-get install pdfmod