Set A4, explicit margins, and change every photo to inline so it flows with the text.
Choose the standard/press-quality preset rather than smallest file — the aggressive preset recompresses images and can drop fonts.
Font size, breaks, photo integrity. Send only after you have looked at every page, not just the first.
Use a font the recipient does not have and the reader substitutes another, changing character widths and pushing the layout apart. Test it: open the exported PDF on a machine that lacks the font and see whether anything moves.
Line spacing, the default printer and paper margins all affect where a page ends. The classic symptom is the last line sliding onto page two. Fix the page size to A4, set explicit margins and insert manual breaks where they matter.
Floating images, text boxes and merged cells are the least stable parts in any conversion. Change images to inline placement and keep tables from spanning pages, and most of the damage disappears.
They lack the font, or use a different reader. Embed fonts, and stop letting Word touch the file after export.
PDF for submission. Word only when they explicitly need to edit.
It is floating and the page box clipped it. Switch to inline placement.
One page for juniors, two is acceptable with experience — but never let page two hold two lines.