Find out what you have
Open the PDF and ask one question: is the text selectable? If it is, the file is a text PDF and it is almost certainly small already. If it is not, the pages are images from a scanner or a camera, and the images are the size. Every useful fix follows from that answer.
Fixes for a scanned document
- Rescan in black and white at 200 to 300 dpi. This can reduce size substantially; check that small text remains readable.
- Turn off colour and photo modes on the scanner or the phone scanner app.
- Drop pages you do not need to send. Remove blank pages, but retain every page the recipient requires.
Fixes for a text document
- Export to PDF instead of printing to image. A PDF exported from Word is a fraction of a scan of the same page.
- Remove large logos and background images from templates; check the preview to ensure any essential branding or labels remain legible.
- Run the browser compressor. It rewrites the file compactly and strips metadata on your device.
What compression cannot do
Browser compression does not re-encode the images inside a PDF, so a scanned document may barely shrink. That is not a fault to work around — it means the file has to be made differently at the source, or split. The limits are five files of 5 MB each and 20 pages per fax.
Splitting is a valid answer
Two faxes of ten pages are better than one fax that fails. Send the first, confirm it was delivered, then send the second, and label both cover sheets so the recipient can see the set is complete. If the carrier rejects a document as too large or too long even after it was accepted here, splitting is the fix the failure reason will suggest as well.