Do online PDF tools upload your file? We measured four.
Most people compressing a contract have no idea whether it stayed on their laptop. It is a checkable fact, not a matter of opinion, so we checked it. Tested 26 August 2026.
What we did
We made a 1,054-byte PDF containing the words “SYNTHETIC TEST FILE” and nothing else, then fed the same file to each tool while recording the size of every request the page sent.
The file size is the whole trick. Plenty of requests fly around any page. Only one of them has a body of exactly 1,054 bytes, and that one is your document.
What happened
iLovePDF/compress_pdf
File was uploaded
POST to api92.ilovepdf.com/v1/upload, body 1,054 bytes
Smallpdf/compress-pdf
File was uploaded
PUT to smallpdf-production-files…r2.cloudflarestorage.com, body 1,054 bytes
PDF24/en/compress-pdf
File was uploaded
POST to filetools20.pdf24.org/client.php?action=upload, body 1,054 bytes
filetity (ours)/pdf-to-jpg
File stayed on the device
Conversion finished and produced a file. No request carried a body.
Smallpdf sends the file to object storage through a signed link rather than to its own API. That is an implementation detail, not a different outcome: the bytes leave the machine either way.
This is not an accusation, and the distinction matters
iLovePDF, Smallpdf and PDF24 are server-side products. They are not hiding it and they are not doing anything wrong by uploading; that is simply how they are built, and it buys them things a browser cannot easily do. This page reports what happens to the file, not what anyone did wrong.
It also cannot see what happens after the upload. Retention, encryption, who can read it, when it is deleted: all real questions, none of them answerable by watching a network tab. A tool that uploads may handle your file impeccably.
The point is narrower. If a document is sensitive enough that you would rather it did not leave your laptop, you can find out in thirty seconds whether it did.
Check any tool yourself, in thirty seconds
You do not need our word or our code. This works on any file tool, including ours.
- 1. Open the tool and let the page finish loading.
- 2. Turn off your wifi, or switch on aeroplane mode.
- 3. Try to use it. A tool that works on your device still works. A tool that uploads cannot.
For the precise version, open developer tools, go to the Network tab, run the conversion, and look for a request whose body is the size of your file. That is the measurement above.
Ours is the one you should be most suspicious of, since we built it and we benefit from the answer. So run the test on our own tool first. The full method, the fixture and its checksum are in how local processing works.
If we got it wrong
Every row is one tool page on one date, and products change. If you build one of these and think we measured it wrong, email support@filetity.com. We will re-run the test, publish what we find, and date the correction, whichever way it goes.