On a laptop, open the browser's Network tab, choose your file and watch for a request about the size of your file: that is an upload. On a phone, use a tool once, switch on airplane mode and try again: a tool that still works is processing on your device.

On a laptop: the Network tab

  1. Open the tool's page in Chrome, Edge or Firefox.
  2. Press F12 (or right-click → Inspect) and open the Network tab. Tick "Preserve log".
  3. Choose your file and press the tool's compress or convert button.
  4. Look at the new requests. Sort by Size, or type method:POST in the filter box (Chrome). A POST or PUT request with a size close to your file's size means the file was sent. Names like upload are another giveaway.

Small requests to ad or analytics domains are normal page traffic, not your file.

On a phone: airplane mode

  1. Use the tool once normally (some tools, including CompressKaro, download their engine on first use).
  2. Switch on airplane mode.
  3. Choose another file and compress it.

If it works, the processing happened on your phone. If it fails or spins, the tool needs its server.

What we found

We ran the Network-tab check on seven PDF compressors on 17 October 2026. iLovePDF, Smallpdf and PDF24 sent the file to their servers as soon as it was chosen; CompressKaro and Pi7 did not. Full results: PDF compressors compared, and our method: how we test.