I placed a 300 dpi image with a transparent background in my ID CS 5.5 (Windows) document, over a dark blue background. Looks great. But when I make a PDF (PDF/X-1a:2001) for the printer there is a very faint square outline around the placed graphic. It’s not a stroke. How do I get rid of it?
You stop using flattened PDF or stop using transparency in your documents. It shouldn’t print on a hi res device but X/1-a is about as dumbed down a PDF as you can get.
Search around for stitching. That’s what you and the printer (who should have known exactly what it was) are seeing. It’s the border where transparency meets the background.
On the very rare occasion that I have to submit artwork to Luddites (always with ads to publications where neither the client nor I have any control) I design accordingly and avoid transparency.
Does it print? It may be a screen artifact. How are you viewing your PDF?
In Acrobat, go to Preferences > Page Diaplay > and uncheck Smooth Line Art, and Smooth Images. Re-view your PDF. Does the faint box disappear?