The Preferences > Appearance of Black does influence the Options for Black on RGB and Grayscale Devices.
Printers, images and monitor screens.
It does not change the values in a PDF or Illustrator file.
A 100% black ink (which always reflects some light) is never as dark as an RGB black (which means the absence of light).
The preferences settings are just for display and export to image purposes.
If you want a darker black in your PDF, use the suggestions for rich black as mentioned in the other replies.
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