PDFtk or The PDF Toolkit is an open source cross-platform tool for manipulating PDF documents. pdftk is basically a front end to the iText library (compiled to Native code using GCJ), capable of splitting, merging, encrypting, decrypting, uncompressing, recompressing, and repairing PDFs.
If pdf is electronic paper, then pdftk is an electronic stapler-remover, hole-punch, binder, secret-decoder-ring, and x-ray-glasses.