Adobe’s PDF reader (Adobe Reader) is certainly a very nice tool for viewing PDFs but it has one annoying “feature” (certainly on Windows): it puts some form of “lock” on the PDF file you are viewing. If you have the file open in Adobe Reader then LuaTeX will see the file is non-writable and will output the PDF in another location using settings in texmf.cnf: TEXMFOUTPUT. This behaviour of Adobe Reader forces you to tediously close and re-open the PDF file each time you update the PDF from a fresh run of LuaTeX.
If, like me, you are doing a lot of “edit–run LuaTeX–view PDF” cycles then you may find that the free Evince PDF viewer can save you a lot of time, and tedium. Evince does not lock the PDF file it is displaying so that LuaTeX can overwrite it and Evince will automatically refresh the display with the new PDF.
For sure, a number of TeX/LaTeX editors have in-built PDF viewers but sometimes you may prefer, or need, to use a standalone PDF viewer; if so, Evince is a nice solution and the Windows version can be downloaded from http://live.gnome.org/Evince/Downloads.