Why "without uploading" matters

Most PDF compressors you find by searching are websites. They work by uploading your file to a server, compressing it there, and sending it back. For a menu or a flyer that may not matter. For a tax return, a medical letter, a contract or a scanned passport, it means a copy of the document sits, at least for a while, on a computer you know nothing about.

Compressing on the phone avoids that. It also works on a plane, in a basement, or anywhere else without an internet connection.

Option 1: Apple's Preview app (iOS 26 and later)

Recent versions of iOS include a Preview app for iPhone. When you export a PDF from Preview, Apple lists options to reduce image resolution for screen viewing and to save embedded images as JPEG, both of which make the file smaller. Apple describes the exact steps in Export or compress PDFs and images in Preview on iPhone.

When this is enough: You have one PDF, it only needs to be somewhat smaller, and your iPhone runs iOS 26 or later. It is already on your phone and costs nothing. Try it first.

Where it falls short: Apple lists these PDF options as switches you turn on or off, without a target size or resolution setting. For a large scan that has to fit under an email limit, that may not be enough.

Option 2: A dedicated on-device compressor

A dedicated app helps most with scanned PDFs. In a scan, every page is a photograph of paper, often stored at a far higher resolution than anyone needs to read it. That is why a few pages from a phone scanner can come to several megabytes. Lowering the image resolution and quality in a controlled way is where the big savings are.

Smart PDF Compressor is a free, offline PDF compressor for iPhone and iPad that helps anyone emailing, uploading or storing a PDF shrink it by up to 95%, without the file ever leaving the device. It is made by Quan Dang, and it needs no account and no subscription.

A real 8-page scan went from 5.6 MB to 669 KB — 88.1% smaller — and stayed readable.

What it adds over the built-in route:

  • It detects whether a PDF is scanned or digital and picks settings to suit it.
  • Light, Standard and Maximum presets, or your own image quality, resolution from 72 to 300 DPI, and grayscale.
  • A side-by-side preview of the original and the compressed file before you save.
  • Batch compression of several PDFs at once, with progress for each.
  • A history of compressed files, searchable by filename and grouped by date.
  • If a PDF is already well optimised, it says so rather than claiming a saving it didn't make.

All of this happens on the device. The app works with no internet connection, uploads nothing, and collects no data.

What to expect

How much a PDF shrinks depends on what is inside it. Scans and photo-heavy PDFs shrink the most. A PDF that is mostly typed text, exported from a word processor, is usually small already and may barely change, whichever tool you use. For those files, a small saving is the honest result, not a failure.

As a rule of thumb, start with the lightest setting that gets the file under the size you need, and check that small print is still legible before you send it.