Compression after the fact always involves some tradeoff. The more reliable way to keep file size down without touching quality is to control what goes into the PDF before it's ever created.
Scan at the resolution you actually need
Most scanning apps default to a higher DPI than necessary for text documents. 150–200 DPI is more than sufficient for readable text, while 300+ DPI is really only needed for documents with fine print or images meant to be reproduced at high quality. Scanning lower to begin with avoids needing aggressive compression later.
Avoid re-scanning already-scanned pages
A PDF made by printing a scanned image and re-scanning it loses quality at every generation, and each pass also tends to add file size rather than reduce it, since compression artifacts from the first scan get re-encoded.
Use vector content where you can
Text typed directly into a document editor stays sharp at any zoom level and takes up minimal space, unlike a screenshot of the same text. Where possible, use editable text rather than an image of text.
Strip out what you don't need before exporting
- Unused embedded fonts — each adds size even if barely used in the document.
- Hidden layers or tracked changes — leftover from editing that don't need to ship in the final file.
- Full-resolution images meant only for on-screen viewing — resize images to the dimensions they'll actually be displayed at before inserting them.
When compression is still the right tool
For PDFs you didn't create yourself — received scans, downloaded reports — compression after the fact is still the practical option, since you can't control how the source file was built. Medium compression is a safe default for most of these cases.