Sorry to disappoint you, but:
- There is no such thing as you are looking for!
If you want code samples on a PDF page to be syntax highlighted, you must look for a tool that does do this within the source document which was used to generate the PDF file from.
There is no tool in the world, neither Free and Open Source Software, nor commercial payware, that lets you edit a PDF and convert the source samples on its pages into properly syntax highlighted parts. (The only thing you can possibly do on this level is adding specific comments -- here you have to manually highlight specific words or sentences with a background color of your choice.)
If you are looking for a toolchain that makes it easy to generate PDFs from scratch containing syntax-highlighted code samples, look at:
Markdown: a very lean text markup language to write the document in (use any text editor you like)
Pandoc: a powerfull Markdown-to-Anything converter. It's a command line tool available for all major OS platforms. Its output may be PDF, HTML, EPUB, LaTeX (all of the previous with syntax highlighting), as well as ODT, DOCX, DocBook (no syntax highlighting supported so far for the last few) and a few more...