Can I export from pdf through Photoshop and back to pdf and keep the color values the same?

Question

I've been sent a pdf which contains text, vector shapes and rasterised images. I would like to take a small area of that pdf, which contains all of these, run that area though Photoshop so I can use a custom photoshop filter on it, and then take the resulting image and put it back in the original pdf.

I can open the pdf in Photoshop. That requires the whole thing to be rasterised. I can specify RBG or CMYK at that stage, and I open it in RGB-8 Adobe-1998 in Photoshop. I crop the image down to just the area I want to change. I then need to save that image out in a format that Acrobat (which I'm using to put the pdf back together) can recognise. Acrobat doesn't seem to like .psd images. However I save the image out of Photoshop, I struggle to get the same color values when I eventually insert it back into my pdf. Color values shift by more than I'd expect, despite everything having embedded profiles.

Is there a workflow that will allow me to rasterise part of a composite pdf, run it through Photoshop, and then embed that rasterised image back in the original pdf, without getting uncontrollable color shifts? I don't mind the odd 1 or 2 shift in 8-bit color value, but I'm getting much more than that. I don't have any control over the color representation of the original pdf but I can set the Photoshop up any way I want. I'm also not restricted to Acrobat. But I do need a controlled and predictable workflow.

Answer 1

After much experiment, I have a workflow that works, although I don't know if that's just because of happenstance.

Exporting anything from the pdf did not produce predictable colors.

Instead, I opened the pdf in Photoshop. That produces a popup to select one page of the pdf to rasterise, and asks for a DPI setting and RGB/CMYK choice. I found that either RGB or CMYK would work, but that CMYK, in my case, gave smaller colour errors.

In Photoshop, I cropped the image down to the area I wanted to modify. I selected the odd shape I was interested in and made everything else in the image transparent. That's so that when I put this rectangular image back in my original pdf, I won't see a faint rectangular outline even if the color is changed one or two units.

When I had finished in Photoshop, I saved the resulting CMYK raster image out as a Photoshop pdf. I then opened that pdf in an Acrobat window, selected it and copied it, and pasted it into another Acrobat window showing my original pdf. I moved and resized the pasted rectangle to cover exactly the part of the pdf I needed to change. That gave me at most a change of one unit in color value.

I also tried saving the Photoshop image as a tiff, which allows Acrobat to insert it as an image, rather than having to open a second pdf window. That does work, but I saw changes of two units in color value.