Probable bug: Color blend with CMYK blending color space rendered incorrectly on iPhone

Question

When a PDF file contains an object with a blend mode (in my case it is exclusion) and a CMYK blending color space, then Acrobat Reader for Windows seems to correctly render the blended color. However, the iPhone version of Acrobat Reader renders the blended color as if the blending color space of the source object were set to an RGB profile.

To demonstrate this, I have created a PDF file containing the same text three times in a row. The middle text has the blend mode set to exclusion and the blending color space set to a CMYK profile. The first and the third text do not have a blend mode. The first text has its color hardcoded to the color that would result from the second text's blend being performed in RGB space, and the third text has the color hardcoded that would result from the blend being perfomed in a CMYK blending color space. Since the second text's blending color space is CMYK, its visible color should match that of the third text.

I have attached the PDF file as well as screenshots taken in both Acrobat Reader on Windows and Acrobat Reader on iPhone. I have an iPhone SE (2020), iOS version 18.6 and Acrobat (Reader?) version 25.08.03.

S_S · Accepted Answer

Hi @maximilian_6016,

Hope you are doing well. Thanks for writing in!

Let me give you an overview of what's happening here:

Acrobat on Windows/macOS uses the full Adobe PDF rendering engine, which correctly respects blend modes in the defined blending color space (CMYK in your case).
Acrobat on iOS/Android uses a lightweight rendering engine optimized for mobile. To simplify and improve performance, it converts page objects into an RGB color space internally before applying blend modes.
Because of that, objects with blend modes like Exclusion, Difference, Overlay, etc. defined in CMYK blending spaces do not display correctly — they behave as if blending were forced into RGB.

Now, when it comes to behavior with Acrobat Reader Mobile:

This is by design/limitation in mobile Acrobat apps.
There is no user setting to force CMYK-accurate blending on iPhone or iPad.
The issue is reproducible and not specific to your PDF.

This is also mentioned in Apple Forums: https://discussions.apple.com/thread/8587377?sortBy=rank

Hope this clarifies your query.

Regards,
Souvik.

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