Direct conversion to grayscale in an overall sense rarely produces great results.
Each image does need to be treated differently when converted to grayscale.
Unfortunately, you will get these inconsistencies, you’ll have to pick one that gives you the best results and hope it’s ok.
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If you’re happy with a lot of the images and you just need to alter a few images (or perhaps them all if you don’t mind doing it?)
In Acrobat Professional
You can use the Touch Up tools to open an image in the the original package, and this can give you control over the grayscale – so you can edit a photograph in photoshop and save it and it will update in the PDF.
https://help.adobe.com/en_US/acrobat/X/pro/using/WS58a04a822e3e50102bd615109794195ff-7c74.w.html#WS58a04a822e3e50102bd615109794195ff-7c70.w
You can also do the similar for vector objects – or even better – replace vector logos with the b/w version if you have you.
https://www.adobe.com/ie/products/acrobat/edit-pdf-images.html
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You could create a b/w version – which would be easy enough to do.
Perhaps it can be done with “Alternate Layouts” but I’m not familiar with that function.
I’d package the current file.
Copy the original InDesign file.
Copy the links and name it “b_w links”
Open the duplicate indesign file.
Relink to the b_w links folder using the Links Panel to redirect to a folder for all the images.
Open all the raster images in photoshop and convert to grayscale (Don’t just choose Image>Mode>Grayscale) – choose Edit>Convert to Profile
Open all the vector images in Illustrator (or equivalent) and replace the colours with a grayscale version https://vectips.com/tricks/four-ways-to-convert-to-grayscale/
Then you’ll need to go select all your swatches in InDesign and delete them all – except “Black, Paper, and Registration” and you’ll be requested to use a swatch to replace all the swatches, choose Black.
Sounds like a long process – but it’s not really.
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Hope you get some help/tips from all that.
let me know if anything isn’t clear.
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