The PDF does not match Excel's print preview — why

Short answer: Excel’s print preview and its PDF export are both drawn against the printer driver currently selected on your machine. The driver reports the usable paper area, and Excel paginates to that. Change the printer — or open the file on a machine with a different default printer — and the page breaks move, which is why the preview you approved and the PDF you got can disagree. Lock the paper size, turn off Excel’s A4-to-Letter substitution, and preview with the same printer selected that you export with.

Why a preview is not a promise

The preview is a rendering of what would print right now, on this printer. Nothing about the workbook stores that decision. So:

Switching the default printer between previewing and exporting moves the breaks.
Exporting on a colleague’s machine produces a different PDF from the identical file.
A driver update can change pagination on a file nobody touched.
Microsoft Print to PDF has its own paper defaults, which is why “print to PDF instead” sometimes produces a different result from “save as PDF”.

None of this is a fault in your file. It is what happens when pagination is a property of the printer instead of the document.

Getting the two to agree

Set the paper size explicitly in Page Layout → Size, rather than accepting the driver default.
Turn off the substitution. File → Options → Advanced → clear “Scale content for A4 or 8.5x11 inch paper sizes”. This alone causes a lot of mismatches.
Select the printer you will export with before previewing — for exports, that usually means Microsoft Print to PDF.
Use fixed scaling, not fit-to-page, if you need identical output across machines. Fit-to-page is recomputed per driver; a fixed percentage is not.
Set margins explicitly rather than leaving them on Normal, which some drivers reinterpret.

When you want the machine to stop mattering

If the same workbook has to produce the same PDF on any machine, the pagination has to come from the document rather than from a driver. That is what a layout pass does: it measures the sheet’s real column widths, chooses the orientation for the sheet, keeps every row whole across page breaks, and resizes a too-wide table to a readable floor or slices it across pages with the label column repeated. The result is decided by the workbook and the paper size, not by whichever printer happens to be default.

CrazySmartPDF works this way, and flags any page that still came out blank or clipped so you see it before your reader does. Windows with Excel 2016+, free on your own files. See how it works.

FAQ

Why do my page breaks move when I save as PDF? Because the PDF export and the on-screen preview can be measured against different paper areas — usually a paper-size substitution or a different selected printer. Lock the size and select the export printer before previewing.

Why does the same file paginate differently on my colleague’s PC? Their default printer reports a different usable area, and Excel repaginates to it. Nothing in the workbook pins the pagination.

Does Page Break Preview show what the PDF will contain? It shows what the current printer would produce. It is a good check, and it is only as stable as that printer selection.

How do I get an identical PDF every time? Fix the paper size, use fixed scaling instead of fit-to-page, set explicit margins — or use a converter that computes the layout from the sheet rather than from the driver.