We run every tool on the same synthetic files, in an automated Chrome browser, and record two things: the size of the file it actually produces for a 200 KB limit, and whether any network request carries the file to a server. We make CompressKaro, so we publish the method and say where other tools do better.
Test files
- Scanned PDF: 2.04 MB, a generated page that looks like a scanned government document. It contains no personal data.
- Photo: 2.03 MB portrait-style JPEG, also generated.
We never upload real documents to any site, and we never sign up or pay.
Output size
On each tool's own "compress PDF to 200 KB" page, or its compress page at the default level when it has no size option, we choose the scan, press the tool's compress button, download the result and measure the downloaded file. When the download can't be automated, we read the size the tool shows on its result screen and say so.
Does the file leave the device?
From the moment the file is chosen, we record every POST, PUT and PATCH request through the browser's DevTools protocol. A request whose body is a PDF, an image, a multipart form or an octet-stream, or whose address is an upload endpoint, counts as an upload. "No upload seen" means none of those appeared; advertising beacons are ignored. Where a vendor states its own handling (servers, deletion times), we quote it with the date.
Page weight
On a 375-pixel phone viewport we load each landing page once, decline cookie banners, and count the JavaScript downloaded and the advertising or tracking hosts contacted before any file is chosen.
Limits of the test
- One file per test. A text-only or 50-page PDF can rank tools differently.
- One run per tool on the test date; websites change. Each page shows when it was last tested.
- "No upload seen" is evidence, not proof that a site never uploads in another flow.
- Automated Chrome: some sites behave differently for automation. Adobe hid its desktop file picker, so Adobe was tested at phone size.
Check any website yourself
Open the site, press F12 (or right-click → Inspect) and go to the Network tab. Choose your file and press compress. If a request appears whose size is close to your file's size, the file was uploaded. On a phone, compress one file normally (some tools download their engine the first time), then switch on airplane mode and compress another: a tool that still works with no connection is processing on your device. Step by step: how to tell if a website uploads your file.