Acrobat grayscale images editied in photoshop convert to rgb when brought back into the pdf file.

Question

After I edit a photo using the object touch up tool in photoshop CS6 and bring it back into acrobat as a grayscale halftone all works fine till I print. It prints on multiple plates. When I edit it again in photoshop the halftone has turned into RGB. It happens every time. I assume there is a color setting in acrobat that is doing the conversion from grayscale to RGB but I don't know where to look. This has worked fine for years and has only recently stopped working. By the way I'm using acrobat pro.

Answer 1

It's really necessary to know by which program you had created the PDF.
Here comes a test PDF for questions like yours, created by InDesign:
http://docs-hoffmann.de/riptest05072013.pdf
Different images:
sRGB, pRGB=ProphotoRGB, CMYK , all with individually embedded profile,
Grayscales: without embedded profiles, because InDesign doesn't embed gray profiles,
Lab: without profile, because Lab is automatically detected and not ambiguuos.
Of course, using so many different images in one PDF is useful only for tests.
An actual test:
Touch Up Object Grayscale and edit in Photoshop. There is no embedded profile
but the image is in PS detected as Grayscale. One has to assign a profile.
Re-imported in the doc, it is a Grayscale JPEG*, before it was a Grayscale ZIP.
In Acrobat 8 one can check the actual content of a PDF by
Advanced > Print Production > Preflight
In newer versions this is still available but somewhere else.
I'm using version 8, because I uninstalled X on purpose.
Best regards --Gernot Hoffmann
*Edited:
As far as I remember, there is no special code for Grayscale JPEGs, it's just a JPEG
with zeros in the Cb Cr channels ... like an R=G=B gray image.