Word offers content immutability with its read-only mode, not sure how much layout change there is, though how much pixel perfection do you need in legal processes that are mostly pure text?
I would recommend taking a good look again. It might answer you why it is preferred in some situation to be typeset in PDF over a format where text could reflow.
About the immutability in Word, it seems optional & not something by design. You can edit any *docx & 'Save it as' back. This feature doesn't absolve immutability as a principal feature.
Nothing happens, embedded fonts feature isn't exclusive to PDFs
If you want literally pixel-perfect layout, you need to use a raster image format like PNG.