Linux probably has a lot more FOSS tools for editing PDF’s than other platforms. ImageMagick will do basic manipulation from the command line. But for a GUI interactive kind of editing, you should look at PDFedit
sudo apt-get install pdfedit
You can do what I’m pretty sure is one of the most often required tasks: Take a page or two out of an existing PDF file and put a page or two into an existing PDF file. Like when you mark up one page of a document, and need to scan the marked-up page and stick it back in the original, replacing the pre-marked up page.
It worked great for me!