Excel won't save as PDF ('Document not saved') - fixes that work

Short answer: When Excel refuses to save as PDF with “Document not saved” or a similar error, the usual causes are a stuck add-in, an old .xls file, the PDF/A compliance option being ticked, or a broken default printer. Those are listed here roughly in order of likelihood. Work through them in that order: start Excel in safe mode to rule out add-ins, save a copy as .xlsx, untick PDF/A in the export options, and set a valid default printer. If you need the PDF right now and Excel still will not cooperate, a browser converter is a reliable fallback.

The fixes, in order

Rule out an add-in. Hold Ctrl while launching Excel to start in safe mode (or launch with excel /safe), then try the export. If it works, an add-in is the culprit: disable add-ins under File, Options, Add-ins and re-enable them one at a time to find it.
Re-save as .xlsx. A legacy .xls (or a corrupt file) often fails PDF export. Use File, Save As, choose the current .xlsx format, then export the new copy.
Untick PDF/A. In the Save As dialog, click Options and clear “ISO 19005-1 compliant (PDF/A).” That option enforces stricter rules and can block an export that otherwise works.
Fix the default printer. Excel’s PDF export leans on printer settings. If your default printer is offline or broken, set a working one (even Microsoft Print to PDF) as default, then try again.
Repair Office. If none of the above helps, run an Office Quick Repair from Windows, Settings, Apps. A damaged install can break the exporter.

Also worth a quick check

Close and reopen the file (or restart Excel) to clear a temporary lock.
Save to a local folder, not a synced or network path, which can intermittently block the write.
Check disk space and the file name; an over-long path or an illegal character in the file name can stop the save.

If you need the PDF right now

When Excel’s exporter simply will not run and you are on a deadline, a browser converter gets you a PDF without touching Excel’s export path. CrazySmartPDF’s free web tool accepts an .xlsx, .xls, .xlsm or .csv, converts it on the server, and deletes the file right after each request. Two honest notes: the web tool uploads the file (so for a strictly no-upload workbook, fix Excel instead), and it converts one file at a time.

FAQ

What does “Document not saved” mean when exporting to PDF? It means Excel could not complete the write, usually because of a stuck add-in, a legacy .xls file, the PDF/A option, or a broken default printer. Work through those four first.

Why does the export work on one file but not another? The failing file is often an old .xls or is mildly corrupt. Save it as a fresh .xlsx and export the copy.

Is there a way to convert without using Excel’s exporter at all? Yes. A browser converter renders the .xlsx independently of Excel’s export path. CrazySmartPDF’s free web tool does this (upload, convert, delete), which is a useful fallback when Excel’s own exporter is the thing that is broken.