PDF was originally conceived as a digital "digital paper" format intended to lock down visual geometry, whereas Microsoft Word is a fluid reflowable document format.
When converting PDF to DOCX, digital native PDFs (created from Word, Google Docs, or InDesign) produce the most pristine editable documents because character positions and font metrics are clearly encoded.
Always review paragraph spacing, table alignments, and header/footer elements after conversion to ensure consistency before submitting your final document.