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

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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.
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.
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