pdftk is definitely going to be your ultimate choice. This small tool, available on all platforms can work wonders.
All the operations are described with examples on the above mentioned page, but I would just list a few commonly used ones:
- Merge PDF Documents
- Split PDF Pages into a New Document
- Rotate PDF Pages or Documents
- Apply a Background Watermark or a Foreground Stamp
- Attach Files to PDF Pages or the PDF Document
etc. This is a very brief glimpse of the operations this software can perform. Installation on all OS’s seems to be very easy.