Ever observed how legal documents, product documentation, books etc., are typeset - with exact placement of comment boxes, US code reference/legal disclaimers/

Ever observed how legal documents, product documentation, books etc., are typeset - with exact placement of comment boxes, US code reference/legal disclaimers/ barcode, precise footnotes or margin notes. Many of those are meant to be also machine readable when printed out. Exactness is a very pressing need. 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.