During the course of working with LuaTeX it can very helpful to explore the internals of the PDF files it generates, especially when using \pdfliteral calls or the LuaTeX PDF API functions (pdf.print() etc). To view the structures inside a PDF file you need tools that will parse and decompress the data and streams inside the PDF. One such utility, and free, is the PoDoFoBrowser. Incidentally, PoDoFo is “… a free, portable C++ library which includes classes to parse PDF files and modify their contents into memory”.

Here’s an example PoDoFoBrowser screenshot:

Non-free option

Another PDF browsing/editing tool, but non-free and Windows only (trial version is free), is PDFTron’s PDF CosEdit, which I also use (registration is $99 plus taxes).

I’m sure there are many others but these are ones I have used.