Answered
change 150 dpi
How to convert to 150dpi and keep layers in a 300dpi PDF file?
How to convert to 150dpi and keep layers in a 300dpi PDF file?
A few ways.. (Assuming you already have layers). Here are Two:
You can apply a Preflight Fixit to do the downsampling for you. There's a built-in one to do exactly what you want.

OR
you could simply Save As Other > Optimized PDF and select a setting to resample your images to 150 for any image over 150.
Neither should affect the layering at all.
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